Quality Control in Veterinary Diagnostic Testing

A review recently published in The Veterinary Journal discusses ways to minimize errors in veterinary diagnostic testing, both with tests run in-house and those sent to diagnostic laboratories.

Melinda Camus, DVM, DACVP, writes that quality control is an emerging subspecialty of veterinary clinical pathology. Because of the increasing availability of point-of-care analyzers, standard laboratory tests are often performed in-house. Quality control is necessary to ensure accurate results. “The approach to quality control in an in-clinic setting initially can be daunting,” she writes. “However, by breaking the testing process into three basic phases (preanalytic, analytic, postanalytic), sources for error can be identified and, hopefully, prevented.”

Preanalytic phase

The preanalytic phase occurs before a diagnostic test is run. Sources of error during this phase include the following:

• Mislabeling or not labeling a specimen tube
• Allowing blood to clot before running a complete blood count
• Accidentally introducing EDTA into a blood sample for chemistry analysis (by inserting a needle into an anticoagulant tube and then into a serum tube)
• Improper preparation of samples for specialized testing

A dog described in the review died because of EDTA contamination of a blood sample. EDTA chelates calcium and magnesium, causing the reported levels to be falsely low. The dog had a serum calcium reading of less than 4.0 mg/dL (reference range, 8.9-11.4 mg/dL). The attending veterinarian administered intravenous calcium gluconate, which caused the patient to develop a cardiac arrhythmia and die. Potassium EDTA also increases the reported level of potassium; the dog’s serum potassium level was greater than 7.0 mEq/L (reference range, 3.6-5.5 mEq/L).

Flow cytometry, polymerase chain reaction for antigen receptor rearrangement, and immunocytochemistry are specialized diagnostic modalities used to identify cell populations (usually neoplastic cells). These modalities require specific sample preparation protocols, which are described in detail in the review.

Analytic phase

The analytic phase is the actual sample analysis. Errors during this phase are associated with the diagnostic equipment (or personnel performing manual tests) and can be caused by the following:

• Expired reagents
• Deterioration of equipment
• Extrapolation of tests from one species to another
• Presence of precipitate or contaminants in stains
• Improper microscope maintenance
• Human error (for example, incorrect interpretation of blood smears or fecal samples)
• Failure to perform proper maintenance and quality control of in-house analyzers
• Improper storage of urine dipsticks (allowing exposure to moisture or light)
• Improper handling of urine dipsticks (dipping them vertically into a sample rather than keeping them horizontal)
• Failure to account for factors like cleaning solutions that can influence results

Bleach and other cleaning products can cause a false-positive reaction for blood on a urine dipstick. Therefore, urine collected from a recently cleaned floor or even pipetted onto a dipstick set down on a recently cleaned counter could yield a false-positive result for blood.

Postanalytic phase

The postanalytic phase occurs after sample testing. These errors involve data handling:

• Incorrect data entry (entering the wrong numbers)
• Assigning results to the wrong patient
• Loss of data

Avoiding errors

Some recommendations to minimize errors include the following:

• Use checklists to help avoid preventable errors
• Use written procedures
• Document personnel training
• Ensure correct sample volumes and processing times
• Document all errors
• Perform and document instrument maintenance
• Use properly stored, in-date reagents
• Repeat tests when results are not compatible with a patient’s clinical presentation
• Back up all data

Creating a quality control system for a veterinary clinic can seem difficult. It may be helpful to consult a clinical pathologist with an interest in quality control. Guidelines are also available online from the American Society of Veterinary Clinical Pathologists.

Dr. Camus chairs the Quality and Laboratory Standards committee of the American Society of Veterinary Clinical Pathologists.

Dr. Laurie Anne Walden received her doctorate in veterinary medicine from North Carolina State University in 1994. After an internship at Auburn University College of Veterinary Medicine, she returned to North Carolina, where she has been in companion animal general practice for over 20 years. Dr. Walden is also a board-certified Editor in the Life Sciences and owner of Walden Medical Writing.