American Society for Veterinary Clinical Pathology
2424 American Lane
Madison, WI 53704

Telephone: +1-608-443-2479
Fax: +1-608-443-2474
email: info@asvcp.org

Principles of Quality Assurance and Standards for Veterinary Reference Values


Decision limits are required for interpretation of laboratory test values. Decision limits may be cut-points or reference intervals. At least one type of decision limit is required to answer relevant clinical problems or questions.

Cut-points are used to differentiate health from disease or, of greater clinical relevance, to differentiate diseases with similar presenting problems. Diagnostic sensitivity and diagnostic specificity are determined at different cut-points. Receiver-operating-characteristic (ROC) curves can be used to help identify optimal cut-points for different clinical needs. If disease prevalence is known, the predictive values of a positive and negative test can be calculated.

Alternatively, clinicians may rely upon reference intervals for interpreting patient test values. Due to the many combinations of factors affecting test results, including instrumentation, reagents, technologists, collection and processing of samples, animal related factors, etc, reference intervals should be confirmed in each laboratory. The reference intervals reported by the laboratory typically are the lowest and highest value expected for the central 95% of similar healthy individuals.

International Federation of Clinical Chemists (IFCC) guidelines are recommended for use when developing reference intervals in veterinary diagnostic laboratories. The IFCC discourages use of the term reference "range" due to the statistical definition of range as a single number, ie, the difference between the upper and lower limit. It is almost impossible for each veterinary diagnostic laboratory to develop and maintain reliable, current, method-related reference intervals for each test and for each subset of animals of potential clinical interest. Frequent changes in technology and the variables of species, breed, age, sex, animal husbandry, diet, and geographic location may create significant differences among laboratories. In spite of this challenge, we recommend, at minimum, that a veterinary laboratory provide the following information to clinicians for every test:

The IFCC recommends a minimum of 120 observations for reference intervals. With this sample size, confidence intervals can be calculated for the lower and upper reference limits independent of whether observations have a normal (Gaussian) or non-normal distribution. The confidence interval around the lower and upper limits for 60 observations with normal distribution is similar to that for 100-120 observations with non-normal distribution. Confidence intervals indicate the reliability of the reference limits and whether a test is able to meet clinical expectations.

Although reference intervals often are developed using fewer observations, until >40 observations are available the best estimates for the central 95% reference intervals are the lowest and highest values observed.
References

  1. Expert Panel on Theory of Reference Values (EPTRV), International Federation of Clinical Chemistry, Scientific Committee, Clinical Section. The theory of reference values. Part 5. Statistical treatment of collected reference values. Determination of reference limits. Clin Chem Acta 1984;137:97F-114F.
  2. Richard Jones and Brian Payne. Data for diagnosis and monitoring. In: Clinical Investigation and Statistics in Laboratory Medicine, Jones R and Payne B (eds), Association of Clinical Biochemists, ACB Venture Publications, London, 1997.