# Documenting QC Strain History: What to Record

By the end of this article you will be able to build an audit-ready history record for every quality control (QC) strain in your microbiology laboratory. You will know which fields belong in the record, which entries attach to each lifecycle event from receipt to disposal, how to document MIC and zone diameter results against published ranges, and how to format the whole thing so an assessor can trace any result on any date back to a specific vial of a specific lot.

You need three things on hand before you start: the certificate of analysis (COA) or datasheet that came with each strain, a controlled document for the record itself (a validated [laboratory information management system](/knowledge/bioinformatics/lims-explained-lab-information-management-systems) (LIMS) module, a locked spreadsheet template, or a paper form under document control), and a copy of your laboratory's standard operating procedure (SOP) for strain handling so the record and the procedure agree with each other.

## Why QC Strain History Matters in Microbiology Quality Control

[Clinical microbiology](/knowledge/diagnostics/microbiology/clinical-microbiology-from-specimen-collection-to-pathogen-identification) laboratories run QC to prove that identification and antimicrobial susceptibility testing (AST) perform as intended, and the standard tool for that proof is a set of well-characterized reference strains such as the ATCC strains. When investigators in Korea set out to build a domestic bank of QC strains, they noted that laboratories routinely rely on ATCC strains for this purpose but face inconvenience from purchase cost and shipping time, and they built their candidate reference strains by testing MICs and zone diameters for eight antimicrobials per strain according to the CLSI M23 method [1]. That study illustrates the two halves of QC strain documentation you must capture: who the strain is, and what it has produced every time you tested it.

The history record is what converts a frozen vial into defensible evidence. If a regulatory assessor or accreditation reviewer asks why your laboratory reported a susceptible result on a given date, the answer runs through a chain: the QC strain lot used that day, its passage count, its storage history, the result it produced that day, and the comparison of that result against the assigned range. A strain with no documented history cannot close that chain, no matter how well it performed.

The identity half of the record has a genomic dimension as well. Shigella flexneri ATCC 12022, a widely used QC and reference strain, had its complete genome assembled and published in 10 scaffolds covering 4.51 Mbp, which means laboratories can anchor their documentation to a molecularly defined strain rather than a name alone [2]. Whole-genome sequencing has also been used to verify, at the strain level, that producers are actually working with the seed strains they claim, which is precisely the question a QC strain history record is designed to answer [3]. Most clinical laboratories will never sequence their QC strains, but the principle carries over: the record should contain enough identity information that a reviewer can confirm the vial in the freezer is the strain the label claims.

## The Three-Tier Strain Bank and the Provenance Chain

Most laboratories organize QC strains in a tiered system. The exact terminology varies between laboratories, so your SOP should define your tiers and the record should follow the SOP. A common arrangement looks like this:

1. **Stock tier (original or master stock).** The frozen vials derived directly from the reconstituted lyophilized culture received from the supplier, or from the original vial received from another laboratory.
2. **Working tier (working stock).** Vials or agar slants prepared from a single stock-tier vial, used for routine monthly QC.
3. **In-use culture.** The plate or slant currently on the bench, subcultured from a working-tier vial and discarded after a defined period.

The record must let any reviewer reconstruct this chain for any vial in the freezer. A vial labeled only "E. coli QC" fails that test. A vial whose record states "working stock W-07, prepared 2024-05-14 from stock vial S-02, lot 70012345, second subculture from reconstitution" closes the loop.

Passage tracking is the part laboratories most often lose. Each transfer of a live culture is a subculture event, and each event is a chance for drift, contamination, or mislabeling. The record should capture every subculture with a date, the operator, the medium, the incubation conditions, and the tier it produced. Establishing explicit acceptance criteria and passage limits is standard laboratory practice, and published QC work consistently depends on controlling strain provenance. The multicenter antifungal work coordinated through the Antifungal Susceptibility Testing Subcommittee of the European Committee on Antimicrobial Susceptibility Testing (AFST-EUCAST), for example, relied on defined QC strains and coordinated testing across centers to establish reliable ranges for yeasts and filamentous fungi [4]. Multicenter reliability of that kind is only achievable when every participating laboratory can document exactly which strain it used.

## What to Record: The Core Fields

### Strain Identity and Source

Record the following for every strain, at the moment it enters the laboratory:

- **Accepted strain designation.** The full designation, for example Escherichia coli ATCC 25922 or Shigella flexneri ATCC 12022 [2]. Use the complete name with the culture collection and number, never a bench nickname.
- **Culture collection and catalog identifier.** Who supplied the strain and under what catalog entry.
- **Derivation of the strain.** Whether it came from a commercial culture collection, a WHO reference panel, or another laboratory. The WHO N. gonorrhoeae reference strains illustrate why this matters. The 2008 WHO reference strain panel was phenotypically and genetically characterized specifically to support global quality assurance and quality control of gonococcal antimicrobial resistance surveillance [5], and the 2016 WHO reference strains received the same treatment with phenotypic, genetic, and reference genome characterization [6]. A strain with that pedigree supports claims a random clinical isolate cannot, and the record is where that pedigree lives.
- **Intended QC role.** Which tests the strain controls. Eubacterium lentum ATCC 43055 was proposed specifically as a new reference strain for quality control of anaerobic susceptibility tests [7], which is a different role from a strain used for routine gram-negative aerobic AST. The record should state the role so a reviewer can tell whether the strain was used within its intended scope.

### Lot Number and Certificate of Analysis

Suppliers assign a lot number to each production batch of a lyophilized strain, and the assigned QC ranges can differ between lots. Record, for each lot:

- The supplier lot number, exactly as printed on the vial.
- The date of receipt and the condition on arrival (lyophilized vial intact, cold pack present, seal intact).
- The assigned QC values printed on the COA, transcribed in full. For an AST strain, that means the MIC range in dilution steps for each agent and the zone diameter range in millimeters for each agent, with the method named.
- The expiration or retest date assigned by the supplier.
- The storage location of the COA itself, or a scanned copy filed in the LIMS, so the record and the certificate can be produced together during an audit.

Transcribing the COA rather than filing it alone is deliberate. The COA is the reference against which every result in the record will be judged, so the ranges belong inside the record, traceable to the lot.

### Passage and Subculture History

For every subculture event, record:

- Date and time.
- Operator initials or identifier.
- Source vial or culture and destination, by tier and vial identifier.
- Medium and incubation conditions (temperature, atmosphere, duration).
- Cumulative subculture count since reconstitution, and the count remaining against your SOP limit.
- Any check performed at transfer, such as purity plating, Gram stain, or colony morphology review.

### Storage and Recovery Conditions

For every freezer entry and retrieval, record:

- Storage location to the box and position level, for example freezer 2, rack B, box 14, position C3.
- Storage temperature and monitoring. Many laboratories store QC strains at minus 70 degrees Celsius or colder for long-term stocks and at 2 to 8 degrees Celsius for short-term slants. Record the temperature regime your SOP defines and note any excursion alarms affecting that location.
- Freeze-thaw history. A working stock intended for single use should record the date it was thawed and whether it was discarded or refrozen. Repeated freeze-thaw cycles damage viability and are a classic source of unexplained QC failures, which is why the record must show them.
- Recovery checks. The date the strain was first revived, the medium used, growth observed, and the date growth was confirmed adequate for testing.

## Lifecycle Events and Their Required Entries

An audit-ready record attaches specific entries to specific events. The table below summarizes the core format. Use it as the field checklist for your template.

| Lifecycle event | Required record entries | Typical evidence attached |
|--|--|--|
| Receipt | Date, supplier, catalog and lot number, vial condition, COA transcription, expiration | COA scan, packing slip |
| Reconstitution | Date, operator, rehydration medium and volume, lot of medium, time to first subculture | Reagent lot record |
| Stock preparation | Date, tier assigned, vial identifiers, number of vials, storage location, cryoprotectant and medium | Freezer map entry |
| Working stock preparation | Date, parent stock vial ID, subculture count, vial IDs and locations | Freezer map entry |
| Routine QC testing | Date, test method, lot of media and panels or disks, incubation conditions, operator, result per agent, comparison against assigned range | Raw instrument or worksheet data |
| Out-of-range event | Date, agent, observed value, assigned range, immediate actions, repeat results, root cause, corrective action, supervisor review | Corrective action report |
| Periodic identity check | Date, method (biochemical, MALDI-TOF, or molecular as your SOP defines), result, operator | Instrument report |
| Disposition | Date discarded or replaced, reason (expiration, passage limit, repeated outlier, contamination), replacement lot and its documentation | Disposal log entry |

Two features make this table audit-ready rather than merely complete. First, every event carries a date and a responsible person, so no entry is anonymous. Second, the disposition event closes the record, which prevents the common audit finding of a QC strain still in use after its documented end of life.

## Documenting QC Test Results and Trend Analysis

### MIC and Zone Diameter Records

For each QC run, record the observed result for every agent tested and compare it against the assigned range for your specific lot. The record should show the comparison explicitly, not leave it implicit. A result line that reads "gentamicin, MIC 1 mg/L, range on COA 0.25 to 1 mg/L, within range, operator J.D." answers the assessor's question on sight.

Understanding where those ranges come from strengthens the documentation. QC limits are set statistically from collaborative data. A classic collaborative study established statistical criteria for selecting QC limits for broth microdilution susceptibility tests across 39 different antimicrobial agents [8]. The Korean reference strain study applied the CLSI M23 method and required that the resulting MIC and zone diameter ranges include at least 95 percent of the data, with zone diameter QC ranges spanning less than 12 mm and MIC QC ranges extending no more than five dilutions [1]. Those constraints exist because a range that is too wide stops detecting problems. Your documentation should reflect the same discipline: record the exact range for your lot, and never widen it locally without a documented justification.

Different organisms and methods carry their own documented QC frameworks, and the record should name the framework that governs each strain. For antifungal susceptibility testing, QC and reference guidelines exist for the CLSI broth microdilution method in document M38-A covering amphotericin B, itraconazole, posaconazole, and voriconazole [9]. For anaerobic bacteria, documented QC recommendations exist for the reference agar dilution procedure, including work on ceftizoxime and cefoxitin comparing results across three NCCLS methods [10], QC guidelines for cefotetan in the reference agar dilution procedure [11], and the proposal of E. lentum ATCC 43055 as a new reference strain for anaerobic susceptibility QC [7]. For gram-positive control strains tested by reference methods, specific MIC QC guidelines have been published for doxycycline [12]. For disk diffusion, a single reference strain has been used to determine gentamicin regression line constants and inhibition zone diameter breakpoints for QC purposes [13]. A well-defined collection of Enterobacteriaceae and Pseudomonas aeruginosa strains has likewise been used to control β-lactam susceptibility testing across multiple Spanish centers [14]. In every case, the record should state which published guideline and which strain the laboratory is following, so a reviewer can retrieve the same source ranges you used.

### Worked Calculation: Rolling Compliance Rate

![Calculation showing 58 results within range out of 60 total runs, giving a compliance rate of 0.9667 or 96.7 percent.](/article-images/illustrations/qc-strain-documentation-audit-ready-records-rolling-compliance-calc.webp)

A useful summary statistic for the record is the proportion of QC results within range over a rolling window. Suppose your E. coli AST strain produced 60 monthly MIC results over five years for a given agent, and 58 fell within the lot-assigned range:

```text
within range = 58
total runs   = 60
compliance   = 58 / 60 = 0.9667 = 96.7%
```

Record the calculation inputs, not only the result, so the number can be recomputed during an audit. The 95 percent inclusion criterion used when such ranges are established [1] gives you a natural benchmark to discuss in your quality review if your own compliance drifts below that level for a specific agent.

### Out-of-Range Results and Corrective Actions

![Flowchart from an out-of-range QC result through repeat testing, classification as random or systematic, corrective action and supervisor review.](/article-images/illustrations/qc-strain-documentation-audit-ready-records-out-of-range-response.webp)

When a result falls outside the assigned range, the record must show a complete response, not just the outlier. A defensible out-of-range entry contains:

- The observed value and the assigned range side by side.
- The immediate action taken, such as repeating the test with the same lot of media and strain, then with a different lot.
- The results of any repeat testing.
- The classification of the event as random or systematic, with the evidence for that classification.
- Corrective actions taken, such as retesting patient results, quarantining a media lot, or replacing the strain stock.
- Supervisor or quality manager review with a signature and date.

The purpose of this level of detail is continuity. If a strain produces an out-of-zone result in March and a similar one in July, the record is what makes the pattern visible, and patterns are what corrective action systems are built to catch.

## An Audit-Ready Record Format and Template

The format matters less than the discipline, but a consistent format makes review fast. The template below works equally well as a LIMS screen layout, a controlled spreadsheet, or a paper form. Bracketed values are illustrative examples. Always replace ranges and identifiers with the values printed on your own certificates of analysis.

```text
QC STRAIN HISTORY RECORD
========================
Record ID:            QC-EC-25922-2024-01
Strain designation:   Escherichia coli ATCC 25922
QC role:              Routine AST QC, gram-negative aerobes
Governing method:     Broth microdilution per [laboratory SOP](/knowledge/diagnostics/molecular/laboratory-sop-writing-guide) ASTM-114
Source:               Commercial culture collection
Catalog number:       [as printed on supplier documentation]
Supplier lot:         [as printed on vial]
Date received:        2024-01-15
Condition on arrival: Lyophilized vial intact, cold pack present
COA on file:          Yes, LIMS attachment QC-EC-25922-2024-01-COA
Expiration:           [as stated on COA]

ASSIGNED QC VALUES (transcribed from COA for this lot)
Agent         Method        Assigned range
[Agent A]     MIC           [x to y mg/L per COA]
[Agent B]     Zone (mm)     [x to y mm per COA]
(Transcribe one line per agent on the COA.)

TIER STRUCTURE
Stock vials:        S-01 to S-06, freezer 2, rack B, box 14
Working stocks:     W-01 onward, prepared one per quarter
Subculture limit:   [as defined in laboratory SOP]

EVENT LOG (append only, one line per event)
Date        Event type        Details                              Operator
2024-01-15  Receipt           Lot received, vial intact            J.D.
2024-01-17  Reconstitution    [medium], [volume], purity check OK  J.D.
2024-01-18  Stock prep        6 x stock vials S-01 to S-06         J.D.
2024-02-01  Working stock     W-01 from S-01, subculture #2        M.R.
2024-02-05  QC run            Results within range, all agents     M.R.
```

The append-only event log is the heart of the audit-ready format. Entries are never edited or deleted. A correction is added as a new line that references the line it corrects, with the reason. This preserves the history exactly as it unfolded, which is what an assessor comparing timestamps wants to see.

For a machine-readable variant, the same structure compresses into a flat file that a LIMS can ingest. Here is the event log as comma-separated values, followed by the output a reviewer would see when the file is opened:

```bash
cat qc_event_log.csv
```

```text
record_id,date,event_type,details,operator
QC-EC-25922-2024-01,2024-01-15,receipt,"Lot received, vial intact",J.D.
QC-EC-25922-2024-01,2024-01-17,reconstitution,"Rehydrated, purity check OK",J.D.
QC-EC-25922-2024-01,2024-01-18,stock_prep,"Vials S-01 to S-06 created",J.D.
QC-EC-25922-2024-01,2024-02-01,working_stock,"W-01 from S-01, subculture #2",M.R.
QC-EC-25922-2024-01,2024-02-05,qc_run,"All agents within range",M.R.
```

```text
record_id,date,event_type,details,operator
QC-EC-25922-2024-01,2024-01-15,receipt,"Lot received, vial intact",J.D.
QC-EC-25922-2024-01,2024-01-17,reconstitution,"Rehydrated, purity check OK",J.D.
QC-EC-25922-2024-01,2024-01-18,stock_prep,"Vials S-01 to S-06 created",J.D.
QC-EC-25922-2024-01,2024-02-01,working_stock,"W-01 from S-01, subculture #2",M.R.
QC-EC-25922-2024-01,2024-02-05,qc_run,"All agents within range",M.R.
```

Five rows in, five rows out. That round-trip check, running the file and confirming the output matches the input row for row, is a simple verification step worth performing whenever a template is first deployed or migrated.

## Step-by-Step Walkthrough: Building a Record for a New Lot

This walkthrough builds a complete record for a new lot of a gram-negative AST QC strain. Follow it in order and you will have an audit-ready file by the end of the first testing cycle.

**Step 1. Open the record before the vial leaves the shipping box.** Assign the record identifier, for example QC-EC-25922-2024-01, and enter the strain designation, supplier, catalog number, and date. Photograph the packing slip and the vial label. If the vial arrives damaged, the record opens anyway and documents the rejection, because a rejected shipment is also a QC event.

**Step 2. Transcribe the certificate of analysis.** Copy every assigned value for every agent into the record, along with the method the values apply to and the lot-specific expiration date. File the scanned COA as an attachment linked to the record identifier. Common error: transcribing ranges from an older lot of the same strain. Ranges can shift between lots, and the audit failure this produces is a result compared against the wrong range, which surfaces during review as "result compared against lot 70012340 range, lot in use 70012345." The fix is to key the transcription against the physical lot number on the vial, not against the strain name.

**Step 3. Reconstitute and document the same day.** Enter the rehydration medium and volume, the operator, the incubation conditions, and the purity check result. Expected output at this step: a growth-confirmed culture and an event log line dated within a day or two of receipt. A long gap between receipt and reconstitution without a documented reason is a frequent audit finding.

**Step 4. Build the stock tier and map it.** Prepare the stock vials per your SOP, assign identifiers, and enter the storage location down to box and position. Update the freezer map in the same session. Expected output: the freezer map query for position "freezer 2, rack B, box 14, positions A1 to A6" returns the six stock vial identifiers and their record ID. If it returns anything else, a labeling error exists somewhere in the chain, and this is the moment to catch it.

**Step 5. Record the first working stock and its passage count.** The event line must name the parent stock vial and the cumulative subculture count. A practical convention is to encode the count in the working stock identifier, for example W-01-P2 meaning working stock 1, second subculture since reconstitution. The count must match the arithmetic of the event log:

```text
Subculture count check
Reconstitution           = subculture 0
Transfer to stock vials  = subculture 1
Preparation of W-01      = subculture 2
Recorded count for W-01  = 2   (matches log, PASS)
```

If the recorded count and the log-derived count disagree, stop and reconcile before any testing proceeds, because every later count inherits the error.

**Step 6. Run the first QC cycle and record results against the transcribed COA.** For each agent, enter the observed value, the assigned range, and the within or out-of-range determination in one line. Expected output for a passing cycle is one clean line per agent with no blank comparison fields. A blank comparison field is the most common formatting defect in QC records, and it is the field an assessor reads first.

**Step 7. Set the review triggers.** Enter the SOP-defined review interval, the passage limit at which the strain will be replaced from a fresh stock vial, and the expiration date. Calendar these against the record identifier. Expected output: a scheduled review entry dated within your SOP interval, for example a quarterly review of the event log and the rolling compliance calculation.

**Step 8. Close the loop at disposition.** When the strain is retired, enter the date, the reason, and the record identifier of the replacing lot. Confirm the freezer map no longer lists the retired vials. Expected output: an event log ending with a disposition line and a freezer map that matches physical reality on the day of an audit.

## Verification, Review, and Retention

A record is audit-ready only if someone periodically verifies it. Build three checks into your laboratory quality management cycle:

1. **Concordance check.** Pick a random QC result from the past month and trace it end to end: result, date, operator, media lot, strain vial, passage count, lot number, COA range. Time the trace. If it takes more than a few minutes, the record has gaps.
2. **Completeness check.** Compare the freezer map against the physical freezer contents and the record list. Every vial present should map to an open record, and every open record should map to a vial or a documented disposition.
3. **Trend review.** At the defined interval, recompute the rolling compliance rate per agent, as shown in the worked calculation above, and review it in the quality meeting. Document the review itself in the record as an event, with reviewer and date.

Retention follows your accreditation and regulatory framework. As a bench rule, retain QC strain records for at least as long as the results they support must be retained, and longer where your quality manual specifies. Retention applies to superseded records as well. When a record is replaced by a new lot's record, the old record is archived, never destroyed, because historical results were judged against the old lot's ranges.

## Common Mistakes and Limitations

- **Ranges compared across lots.** Results from lot B judged against lot A's ranges. The fix is lot-specific transcription at receipt and a LIMS rule that ties each result line to a lot number.
- **Passage counts reconstructed from memory.** Counts written months after the fact always drift. Record the count at the moment of transfer, in the same entry as the transfer.
- **The unrecorded subculture.** A technologist streaks the QC strain onto a plate for convenience, uses it two days later, and no event is logged. That plate is a subculture and belongs in the log. Bench habits, not templates, are where audit findings are born.
- **Anonymous corrections.** Erasing or overwriting an entry instead of appending a correction. The append-only log with explicit correction lines is the standard answer.
- **Nickname drift.** A strain recorded as "EC QC" on the worksheet, "E. coli 25922" in the LIMS, and "ATCC 25922" on the freezer label. Search fails, and the linkage between record and vial breaks. Use the full designation everywhere, and let short forms exist only as controlled aliases in the LIMS.
- **COA filed but never transcribed.** A scanned certificate in a shared drive is not part of the record until the record references it and carries the values. Auditors ask to see the ranges inside the record.
- **Missing disposition events.** Retired strains that still appear on the freezer map. Close every record with a disposition line.
- **Confusing identity with performance.** A strain that performs within range is not thereby proven to be the correct strain. Identity checks such as MALDI-TOF or biochemical profiling, performed at the interval your SOP defines, address identity. Performance records address behavior. The record needs both.

One limitation belongs in the open. This article describes record-keeping practice at the level of standard laboratory quality management. The specific passage limits, review intervals, and retention periods in force in your laboratory are set by your SOP and your accrediting body, and individual implementation questions belong with your laboratory's quality manager.

## Frequently Asked Questions

### How many subcultures are allowed before a QC strain must be replaced?

Your laboratory SOP defines the limit, and the record exists to enforce it. The standard practice is to prepare a deep stock tier on receipt, draw working stocks from a single stock vial, and track every subculture in the event log so the count against the SOP limit is always visible on the record.

### What is the difference between a QC strain and a reference strain?

A QC strain is used routinely to check that a test system performs within assigned limits on the day. A reference strain carries a higher level of characterization, often phenotypic and genetic or genomic, and anchors the assignment of those limits. Many strains, such as well-characterized ATCC and WHO panel strains, serve both roles, and the record should state which role applies to each use.

### Can I keep QC strain records in a spreadsheet instead of a LIMS?

Yes, if the spreadsheet is under document control, access-restricted, versioned, backed up, and maintained in the append-only style described above. The format is not what makes a record audit-ready. The completeness, traceability, and change discipline are.

### What should I do when a QC result falls outside the published range?

Record the observed value against the assigned range immediately, repeat the test per your SOP, classify the event as random or systematic based on the repeat and any concurrent lot changes, and document corrective action and supervisor review. The record entry, not the memory, is the corrective action system's input.

### How long must QC strain records be kept?

Retain them at least as long as your accrediting body and quality manual require for the patient results they support, and archive superseded records rather than destroying them. The archive matters because historical results were judged against historical ranges.

### Do I need to document the genome sequence of my QC strain?

Most clinical laboratories do not, and published QC frameworks rest on phenotypic performance against assigned ranges. Document the source, lot, and characterization level the supplier and the relevant guideline provide. Genomic verification becomes relevant when provenance is genuinely in question, since complete genome sequencing has been used to confirm strain-level identity in reference and seed strain programs.

### What goes on the label of a frozen QC stock vial?

At minimum the strain designation, the tier and vial identifier, the parent record identifier, the lot number, the subculture count, the date prepared, and the operator initials. The label is a pointer into the record, not a substitute for it, so keep the label short and keep the detail in the log.

### How do I document a QC strain received from another laboratory rather than a culture collection?

Record the donor laboratory, the donor's own strain identifier and passage history as far as it is disclosed, the transfer date and conditions, and any identity and performance verification your laboratory performs on arrival. Treat the first in-house QC cycle as a qualification run against the applicable published ranges before the strain enters routine use, and record that qualification as its own event in the log.

## Sources

1. [Establishing Quality Control Ranges for Antimicrobial Susceptibility Testing of Escherichia coli, Pseudomonas aeruginosa, and Staphylococcus aureus: A Cornerstone to Develop Reference Strains for Korean Clinical Microbiology Laboratories](https://pubmed.ncbi.nlm.nih.gov/26354353/)
2. [Genome Assembly of Shigella flexneri ATCC 12022, a Quality Control Reference Strain](https://pubmed.ncbi.nlm.nih.gov/25359907/)
3. [High-quality genome assembly and annotation of live animal vaccine bacteria strains in South Korea](https://pubmed.ncbi.nlm.nih.gov/40898044/)
4. [Multicentre determination of quality control strains and quality control ranges for antifungal susceptibility testing of yeasts and filamentous fungi using the methods of the Antifungal Susceptibility Testing Subcommittee of the European Committee on Antimicrobial Susceptibility Testing (AFST-EUCAST](https://pubmed.ncbi.nlm.nih.gov/17697001/)
5. [Phenotypic and genetic characterization of the 2008 WHO Neisseria gonorrhoeae reference strain panel intended for global quality assurance and quality control of gonococcal antimicrobial resistance surveillance for public health purposes.](https://pubmed.ncbi.nlm.nih.gov/19318360/)
6. [The novel 2016 WHO Neisseria gonorrhoeae reference strains for global quality assurance of laboratory investigations: phenotypic, genetic and reference genome characterization.](https://pubmed.ncbi.nlm.nih.gov/27432602/)
7. [Eubacterium lentum ATCC 43055, a new reference strain for quality control of anaerobic susceptibility tests](https://pubmed.ncbi.nlm.nih.gov/2229368/)
8. [Statistical criteria for selecting quality control limits for broth microdilution susceptibility tests with 39 different antimicrobial agents. Collaborative Antimicrobial Susceptibility Testing Group.](https://pubmed.ncbi.nlm.nih.gov/2612129/)
9. [Quality Control and Reference Guidelines for CLSI Broth Microdilution Susceptibility Method (M38-A Document) for Amphotericin B, Itraconazole, Posaconazole, and Voriconazole](https://pubmed.ncbi.nlm.nih.gov/16207990/)
10. [Ceftizoxime and cefoxitin susceptibility testing against anaerobic bacteria: comparison of results from three NCCLS methods and quality control recommendations for the reference agar dilution procedure.](https://pubmed.ncbi.nlm.nih.gov/3480787/)
11. [Quality control guidelines for testing cefotetan in the reference agar dilution procedure for susceptibility testing of anaerobic bacteria](https://pubmed.ncbi.nlm.nih.gov/2643621/)
12. [MIC quality control guidelines for doxycycline when testing gram-positive control strains by the reference methods.](https://pubmed.ncbi.nlm.nih.gov/15582304/)
13. [Analysis of a single reference strain for determination of gentamicin regression line constants and inhibition zone diameter breakpoints in quality control of disk diffusion antibiotic susceptibility testing](https://pubmed.ncbi.nlm.nih.gov/6818249/)
14. [Quality Control for β-Lactam Susceptibility Testing with a Well-Defined Collection of Enterobacteriaceae and Pseudomonas aeruginosa Strains in Spain](https://pubmed.ncbi.nlm.nih.gov/12734226/)