Establishing Reference Intervals in Veterinary Clinical Chemistry: Principles, Methods, and Applications

Introduction

Reference intervals (RIs) are essential interpretative tools in veterinary clinical pathology, providing the physiological benchmark against which laboratory results from individual animals are compared [1]. Without appropriate RIs, the clinical significance of biochemical and hematological measurements cannot be reliably assessed, potentially leading to misdiagnosis or delayed intervention [1, 2]. The process of RI generation involves a systematic workflow from reference population selection through statistical computation, and must account for the substantial biological variation that exists across species, breeds, age groups, and environmental conditions [3, 4].

The American Society for Veterinary Clinical Pathology (ASVCP) and the Clinical and Laboratory Standards Institute (CLSI) have published detailed guidelines for RI determination [5, 6, 7, 8, 9, 10, 11, 12, 3, 13, 14, 15, 4]. These protocols emphasize rigorous inclusion and exclusion criteria, adequate sample size, proper handling of preanalytical variables, appropriate statistical analysis, and transparent reporting of reference limits with confidence intervals [16, 3, 17, 18, 19]. This article provides a comprehensive review of the theoretical and practical aspects of establishing RI in veterinary clinical chemistry, drawing on a wide range of species-specific studies.

Definition and Clinical Importance

A reference interval is defined as the central 95% range of values obtained from a defined population of healthy individuals, typically bounded by the 2.5th and 97.5th percentiles [1, 18]. In veterinary practice, RIs are used to interpret results from hematology, serum biochemistry, blood gas analysis, and specialized assays such as protein electrophoresis and acute-phase protein measurements [20, 21, 19]. The clinical utility of RIs depends on the appropriateness of the reference population to the patient being evaluated: factors such as species, breed, age, sex, physiological state, environmental conditions, and analytical methodology must be matched as closely as possible [22, 23, 12, 3, 24, 4].

RIs are particularly critical in wildlife conservation medicine, where baseline physiological data are often absent for endangered or poorly studied species [5, 6, 25, 26, 8, 9, 10, 27, 11, 12, 14, 15, 28]. For example, RIs have been established for free-ranging African elephants (Loxodonta africana) [8], Hawaiian monk seals (Neomonachus schauinslandi) [5], Spix's macaws (Cyanopsitta spixii) [25], and Temminck's pangolins (Smutsia temminckii) [9], providing essential tools for health monitoring and conservation decision-making.

Selection of Reference Individuals

The selection of a reference population is the foundation of any RI study. The population must be clearly defined and should comprise individuals that are clinically healthy based on thorough history, physical examination, and, where possible, ancillary diagnostic tests [22, 5, 6, 26, 11, 3, 29, 30, 13]. Inclusion and exclusion criteria must be established a priori and documented [1, 10]. Common exclusion criteria include evidence of disease, recent medication or vaccination, abnormal findings on physical examination, and laboratory results suggestive of subclinical pathology [22, 29, 14].

For domestic species, large cohorts can be sourced from breeding facilities, research colonies, or clinical practice populations [23, 3, 17, 18, 2, 24, 4]. For example, Zhu et al. utilized 1,163 serum samples from five geographically distinct donkey farms to establish biochemistry RIs for Chinese local donkeys [3]. In contrast, studies on wildlife often rely on smaller sample sizes due to logistical constraints, but must still adhere to the same rigorous health assessment protocols [5, 6, 25, 9, 27, 14].

Preanalytical and Analytical Considerations

Preanalytical factors that can influence measurand values include diet, fasting status, time of day, season, capture method, sample type (serum vs. plasma), anticoagulant, storage temperature, and transport time [22, 7, 9, 12, 29, 14, 19]. For instance, serum protein fractions in barren-ground caribou show pronounced seasonal variation, necessitating season-specific RIs [7]. Similarly, differences between free-living and rehabilitated pangolins have been attributed to nutrition and hydration status [9].

Analytical methodology is another major source of variation. Different analyzers, reagents, and assay principles can yield systematically different results for the same measurand [9, 31, 2]. Therefore, RIs must be validated for each specific instrument and laboratory [23, 2]. Method comparison studies are essential when introducing new analyzers or reagents, as demonstrated by Pacumio et al. for beta-hydroxybutyrate measurement in bearded dragons [31] and by Teixeira et al. for canine biochemistry on the Respons920 analyzer [23].

Statistical Methods for Reference Interval Calculation

The recommended approach for RI calculation follows the ASVCP guidelines and depends on the sample size and distributional properties of the data [16, 3, 17, 19].

Parametric methods assume a Gaussian distribution (or a transformed distribution) and calculate the mean ± 1.96 standard deviations to define the central 95% interval [18]. These methods are efficient when sample sizes are large and the normality assumption holds.

Nonparametric methods, which make no distributional assumptions, rank the observations and use the 2.5th and 97.5th percentiles as the reference limits [1, 3, 17]. This approach is recommended for sample sizes of at least 120 individuals to achieve reliable estimates, though smaller samples can be used with caution and appropriate confidence intervals [6, 19].

Robust methods, such as the Horn algorithm based on Box-Cox transformation and robust estimation of location and scale, are valuable when data contain outliers or are moderately non-Gaussian [16, 19].

Outlier detection is a critical step. Common methods include the Tukey interquartile range rule, the Dixon range statistic, and visual inspection of histograms and Q-Q plots [7, 8, 16, 3]. After removal of outliers, the remaining data are used to compute RIs and their 90% confidence intervals around the lower and upper reference limits [18, 19].

Partitioning of Reference Intervals

When biologically meaningful subpopulations (e.g., males vs. females, juveniles vs. adults) are present, the need for separate RIs can be evaluated using the Harris and Boyd partitioning criteria [23, 12, 3, 4]. Partitioning is justified if the differences between subgroups are statistically and clinically significant. In many species, age and sex effects are observed but may not be large enough to warrant separate RIs [23, 3, 4]. For example, in Chinese donkeys, farm origin was the primary source of variation, while age and sex effects were not sufficient for partitioning [3]. Conversely, in Lusitano horses, female-specific RIs are recommended for certain enzymes [24], and in guinea pigs, age-related changes in glucose, albumin, and alkaline phosphatase support separate adult and juvenile intervals [32].

Species-Specific Reference Intervals: Examples from the Literature

The following table summarizes selected RI studies from the provided literature, highlighting key species and the parameters established.

Workflow for Establishing Reference Intervals

The following Mermaid diagram illustrates the stepwise process recommended for generating RIs in veterinary clinical chemistry.

flowchart TD
    A[Define Reference Population], > B[Health Assessment]
    B, > C{Meet Inclusion Criteria?}
    C, >|Yes| D[Collect Samples]
    C, >|No| E[Exclude]
    D, > F[Preanalytical Handling]
    F, > G[Analytical Measurement]
    G, > H[Data Review]
    H, > I[Outlier Detection]
    I, > J[Assess Distribution]
    J, > K[Select Statistical Method]
    K, > L[Calculate Reference Limits & 90% CI]
    L, > M{Partitioning Needed?}
    M, >|Yes| N[Separate RIs per Subgroup]
    M, >|No| O[Single RI]
    N, > P[Validate & Report]
    O, > P
    P, > Q[Clinical Application]

Challenges and Limitations

Several challenges complicate the generation of valid RIs in veterinary medicine. Sample size limitations are common, particularly for exotic and wildlife species, and may necessitate reliance on robust or nonparametric methods despite reduced statistical power [6, 9, 27, 14]. The definition of "health" can be ambiguous; subclinical infections or early disease may not be detected by routine screening, potentially introducing bias [7, 12, 29]. Additionally, analytical bias between different laboratory platforms can limit the transferability of RIs, reinforcing the need for laboratory-specific validation [9, 31, 2].

Seasonal and geographic variation also affects many analytes. In free-ranging species, changes in diet, reproductive status, and environmental stressors can shift reference limits, as shown in caribou serum proteins [7], koala biochemistry [12], and seabird hematology [14]. Comprehensive RI studies should therefore consider temporal and spatial factors and, where possible, establish partitioned RIs.

Frequently Asked Questions

What are reference intervals in veterinary clinical chemistry?

Reference intervals are the central 95% range of laboratory test values derived from a clinically healthy reference population, used as a benchmark to interpret individual patient results [1, 18].

How are reference individuals selected for a veterinary RI study?

Reference individuals are selected based on stringent inclusion and exclusion criteria, including a thorough health assessment by history, physical examination, and, when feasible, complementary diagnostic tests to confirm the absence of disease [22, 5, 6, 29, 30, 14].

What statistical methods are commonly used to calculate RIs?

The three main approaches are parametric (mean ± 1.96 SD for Gaussian data), nonparametric (2.5th and 97.5th percentiles), and robust methods (e.g., Horn algorithm) [1, 16, 3, 18, 19]. The choice depends on sample size and data distribution.

When should separate RIs be established for subpopulations?

Separate RIs should be considered when factors such as age, sex, breed, or physiological state produce clinically significant differences in analyte values, as determined by partitioning criteria like the Harris and Boyd method [23, 12, 3, 4].

Can RIs from one laboratory be used on another analyzer?

No, RIs are generally instrument- and laboratory-specific due to differences in analytical methods, reagents, and calibration [9, 31, 2]. Validation or de novo generation is recommended when changing analytical platforms.

How should preanalytical variation be managed in RI studies?

Preanalytical factors such as fasting status, sample type, anticoagulant, storage, and time to analysis must be standardized and documented, as they can significantly affect measurand concentrations [22, 7, 9, 12, 14, 19].

Conclusion

Establishing robust reference intervals is a foundational element of veterinary clinical chemistry. Adherence to standardized protocols for reference population selection, sample handling, analysis, and statistical computation ensures that RIs are reliable tools for clinical decision-making. The growing literature across a diverse range of species, from domestic dogs and cats to endangered wildlife, underscores the importance of context-specific RIs. Continued efforts to generate RIs for understudied species and to evaluate the impact of biological and methodological variables will further enhance the accuracy of veterinary diagnostics.

References

[1] Quagliardi M, Galosi L, Rossi G, et al. Reference Intervals (RIs) in veterinary medicine. ACTA IMEKO. URL: https://www.semanticscholar.org/paper/21be70a49d5436cbfe1239b6af167a051bc97405

[2] Chung S, Chang L, Cheng T, et al. ESTABLISHING IN-HOUSE REFERENCE INTERVALS FOR DOGS IN VETERINARY CLINICS. Journal. URL: https://www.semanticscholar.org/paper/7fb14a7866f0b906b2f3f81040811984ed9bc083

[3] Zhu Q, Khan MZ, Jing Y, et al. Establishment and evaluation of hematological and serum biochemical reference intervals in Chinese local donkey populations. BMC Veterinary Research. URL: https://www.semanticscholar.org/paper/1e29fb9c402257213c4fe6e6e8dda5aaa73092c1

[4] Præstegaard KF, Winther-Larsen A, Kousholt BS. Hematological reference intervals for Danish crossbred Landrace Yorkshire Duroc (LYD) pigs used in biomedical research. Acta Veterinaria Scandinavica. URL: https://www.semanticscholar.org/paper/9b35321092ca8232f395890aa66a37efe2943507

[5] Kaufman AC, Robinson S, Borjesson D, et al. ESTABLISHING HEMATOLOGY AND SERUM CHEMISTRY REFERENCE INTERVALS FOR WILD HAWAIIAN MONK SEALS (NEOMONACHUS SCHAUINSLANDI). Journal of zoo and wildlife medicine. URL: https://www.semanticscholar.org/paper/b05b9a9adeb47e75148167ec1aeea9e024bcd080

[6] Kim M, Wut Hmohn ZZ, Jang W, et al. Hematologic and clinical chemistry reference intervals for six species of wild birds frequently rescued in the Republic of Korea. Frontiers in Veterinary Science. URL: https://www.semanticscholar.org/paper/8c979d93c19fa7db9f652bbcf27a8489e0807304

[7] Daleo MJ, Lieske C, Cray C, et al. Establishing serum protein electrophoresis reference intervals in free-ranging barren-ground caribou (Rangifer tarandus granti). Frontiers in Veterinary Science. URL: https://www.semanticscholar.org/paper/cc9219f9131f6e3edc9eb597cc5d66d3e6888f54

[8] Steyrer C, Miller MA, Hewlett J, et al. Reference Intervals for Hematology and Clinical Chemistry for the African Elephant (Loxodonta africana). Frontiers in Veterinary Science. URL: https://www.semanticscholar.org/paper/307ae40bd0756b7f1fb9d16826fa6930fd219be4

[9] Hooijberg E, Lourens K, Meyer L. Reference Intervals for Selected Hematology and Clinical Chemistry Measurands in Temminck's Pangolin (Smutsia temminckii). Frontiers in Veterinary Science. URL: https://www.semanticscholar.org/paper/1534e31596224e72b0a57b4ef14a0f9539c78157

[10] Broughton H, Wolf K, Anderson KM. Characterising Reference Intervals in Clinically Normal Red Wolves (Canis rufus): A Baseline for the Detection of Clinical Disease. Veterinary Medicine and Science. URL: https://www.semanticscholar.org/paper/d1a9a27876750f19b8e64614d17208377accc0a2

[11] Machado PC, Motta Lima CF, Ramos PL, et al. Establishing hematologic and biochemical reference intervals for the endangered Lear's macaw (Anodorhynchus leari). Veterinary clinical pathology. URL: https://www.semanticscholar.org/paper/881ff258fd62cefdfbb18046a73f30f279757ab1

[12] Font M, Woolford L, Jaensch S, et al. Biochemical Reference Intervals of Free‐Ranging Koalas (Phascolarctos cinereus) in South Australia. Veterinary clinical pathology. URL: https://www.semanticscholar.org/paper/6bc3e7076cdb989f6bf9426ef1b36d71b981bd8c

[13] Hayakijkosol O, Gerber K, Miller DJ, et al. Development of reference intervals for serum biochemistry and haematology of juvenile green sea turtles (Chelonia mydas) in a Thai rehabilitation centre. Australian Veterinary Journal. URL: https://www.semanticscholar.org/paper/93df11d616d1adde90f82c1d94c5cb742ce93dfa

[14] Maçaira J, Felix MR, Alcântara AO, et al. Hematological and biochemical reference intervals for populations of seabird species from an Archipelago in Rio de Janeiro, Brazil. Frontiers in Veterinary Science. URL: https://www.semanticscholar.org/paper/7e64f985926a447fb553f649de2996a01e51872f

[15] Muliya SK, Bindhani UT, Kawlni L, et al. Reference intervals for hematology, serum biochemistry and blood gas parameters in Indian elephants (Elephas maximus indicus) under human care. Frontiers in Veterinary Science. URL: https://www.semanticscholar.org/paper/ba026ee2a793462c2752f1d9e6703b238d62dd92

[16] Conrado FO, Leveille-Webster C, Berlin N, et al. Hemorheology Reference Intervals in Healthy Dogs Using the MIZAR Analyzer. Veterinary clinical pathology. URL: https://www.semanticscholar.org/paper/025992b59a792a31d5305251b1e899d898ab3db5

[17] Massat MJ, Myles M, DeJong K, et al. Plasma chemistry reference intervals for adult multi-drug-resistance gene deficient Collies. Veterinary clinical pathology. URL: https://www.semanticscholar.org/paper/3461fc1475fdec710ba449cc2b4a329b0a463dfd

[18] de Oliveira CS, de Lima Francisco C, Marques da Silva DC, et al. Reference intervals for blood biochemical parameters in growing male water buffaloes (Bubalus bubalis). The Veterinary Journal. URL: https://www.semanticscholar.org/paper/9957969b4ba08e0cd42375fdac41d93e00d28a13

[19] Thomas S, Moore AR, Pollak MB, et al. De Novo Serum Biochemistry and Electrophoretic Reference Intervals for Jamaican Fruit Bats (Artibeus jamaicensis). Journal of the American Association for Laboratory Animal Science. URL: https://www.semanticscholar.org/paper/ded41e6a4fa7e268d0e2398ef47cc

[20] Park S, Taili I, Kim S, et al. Serum amyloid A reference intervals in raccoon dogs (Nyctereutes procyonoides koreensis): Implications for detecting inflammation. Journal of Veterinary Medical Science. URL: https://www.semanticscholar.org/paper/6c76d2d01e8a05b919291e5a847fbfeecbbb3463

[21] Lee C, Lee SL, Kim E, et al. Whole Blood Viscosity Reference Intervals and Its Correlation with Hematology and Serum Chemistry in Cats Using Scanning Capillary Method. Animals. URL: https://www.semanticscholar.org/paper/84b272a2f5998e7243eb07b3dd1789232d393422

[22] Ventrella D, Dondi F, Barone F, et al. The biomedical piglet: establishing reference intervals for haematology and clinical chemistry parameters of two age groups with and without iron supplementation. BMC Veterinary Research. URL: https://www.semanticscholar.org/paper/205475266589a622b90df48754edcbdc2558fba6

[23] Teixeira B, Silvestre-Ferreira AC, Maia A, et al. Establishing biochemical reference intervals in dogs using the Respons920® analyser. Croatian veterinary journal. URL: https://www.semanticscholar.org/paper/cad761f486e94eae32f68257668aec8e70f78525

[24] Pires MJ, Cotovio M, Queiroga F, et al. Reference Intervals for Biochemical Analytes in Clinically Healthy Adult Lusitano Horses. Veterinary Sciences. URL: https://www.semanticscholar.org/paper/d9beab0eff1d8b7390ca60402885d66a9344de3d

[25] Farault J, Marcuk V, Scholtyssek K, et al. Establishing hematological and biochemical reference intervals in Spix's macaws as tools for environmental and conservation biomonitoring. Science of the Total Environment. URL: https://www.semanticscholar.org/paper/6a819c4691f2270d7b3637602b2f7efed60a77f0

[26] Park JK, Kim JB, Do Y. Reference Intervals in Combined Veterinary Clinical Examinations of Male Black-Spotted Pond Frogs (Pelophylax nigromaculatus). Animals. URL: https://www.semanticscholar.org/paper/a0aada03178a9c8bc4a96f707e64a45c295ad14f

[27] Dannemiller NG, Knutson KA, Arnold JE, et al. PRELIMINARY HEMATOLOGY AND BIOCHEMISTRY REFERENCE INTERVALS IN JUVENILE BLACKTIP SHARKS (CARCHARHINUS LIMBATUS) IN THE GALÁPAGOS MARINE RESERVE. Journal of zoo and wildlife medicine. URL: https://www.semanticscholar.org/paper/145b486ed0c5b7ee5130d30b979d9897fe876c4c

[28] Fernández-Sanz H, Seminoff JA, Mancini A, et al. Hematology of East Pacific green turtles (Chelonia mydas) in the Southern Gulf of California, Mexico: reference intervals of healthy turtles and intra-population comparisons. Journal of Comparative Physiology B. URL: https://www.semanticscholar.org/paper/7609f23b983901064cecb3fce1d1d684d0f7ce4e

[29] Carrillo-Muro O, Rodríguez-Cordero D, Hernández-Briano P, et al. Enzymic Activity, Metabolites, and Hematological Responses in High-Risk Newly Received Calves for “Clinical Health” Reference Intervals. Animals. URL: https://www.semanticscholar.org/paper/dc82dc8891cb05c9dd6c5f4ac60b0c43f85651b2

[30] Nunes NJS, Valle SF, Okano FY, et al. Reference intervals for bone marrow cells in juvenile and young adult cats bone marrow cytology. Veterinary clinical pathology. URL: https://www.semanticscholar.org/paper/3a9103dcdeee1e87420253df86c39fe0decf73df

[31] Pacumio L, Tarbert DK, Ammersbach M, et al. Method Comparison and Reference Intervals of β-hydroxybutyric Acid Measurements Using a Veterinary Point-of-care Ketone Meter and a Reference Laboratory Analyzer in Central Bearded Dragons (Pogona vitticeps). Journal of Herpetological Medicine and Surgery. URL: https://www.semanticscholar.org/paper/9ac4ee1923af4773f85c50953dc9ebb80f8851b4

[32] Genzer S, Huynh T, Coleman-McCray J, et al. Hematology and Clinical Chemistry Reference Intervals for Inbred Strain 13/n Guinea Pigs (Cavia Porcellus). Journal of the American Association for Laboratory Animal Science. URL: https://www.semanticscholar.org/paper/3833b29fbf1959732983384da7596fc6d59de6f2

[33] Cummings AM, Cray C, Montiani-Ferreira F, et al. Generation of Reference Intervals and Evaluation of Seasonal Variation in Clinical Pathology Parameters of Backyard Laying Hens. Journal of avian medicine and surgery. URL: https://www.semanticscholar.org/paper/6aa8126595cc098e3a301f65b7b5d0a8948e9100

[34] Lorsunyaluck B, Kaewthumchai O, Tinnakorn C, et al. The first study of reference intervals for blood biochemistry of healthy pet Pacific parrotlets (Forpus coelestis) in Thailand. Journal of Advanced Veterinary and Animal Research. URL: https://www.semanticscholar.org/paper/50a4c7284a2171dbbbf657f3556bd9a8473c83ec

[35] Okulmuş Ç, İçil N, Turkyılmaz O, et al. Clinical chemistry reference intervals for swiss albino strain mice commonly used in scientific studies. Comparative Clinical Pathology. URL: https://www.semanticscholar.org/paper/670b326a2dcf64683b059091ec620de4e7f6ad19