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Factors impacting blood sample results in healthy cats

Norsworthy GD, Cook AK, Lanier CJ. Impact of preheparinization and sample volume on routine hematology findings in healthy cats. J Feline Med Surg [Internet]. 2020 Jun 1.

Collection of blood samples in cats may be difficult, especially when multiple tests need to be run in small or debilitated patients. Various blood tests need to be run on different sample types; for example a complete blood count is generally performed on blood anticoagulated with EDTA, while biochemistry samples may be run on heparinized blood. In pediatric or critically ill patients, or in patients where repeated sampling is needed, it may be difficult (either technically or due to iatrogenic anemia) to collect multiple vials of blood. While contamination of biochemistry samples with EDTA will lead to inaccurate results (ie hypocalcaemia, hyperkalemia), it may be feasible to perform hematology studies on previously heparinized blood.

The purpose of this study was to determine if preheparinization of EDTA samples affected CBC parameters on an in-house hematology analyzer, to determine the ideal volume of blood to use, and further to determine if preheparinization affected leukocyte morphology. The study was designed as a prospective single blinded noninferiority study.

Twenty healthy, client owned cats were used for this study. 5mL of blood were collected via jugular venipuncture. Samples were placed into tubes with a recommended volume of 1.3mL. Samples were divided as follows:

  • 1.3mL into a 1.3mL EDTA tube (1.3mL EDTA)
  • 0.5mL into a 1.3mL EDTA tube (0.5mL EDTA)
  • 1.3mL into a 1.3mL heparin tube, then all 1.3mL into a 1.3mL EDTA tube (1.3mL Hep+EDTA)
  • 1.3mL into a 1.3mL heparin tube, then 0.5mL into a 1.3mL EDTA tube (0.5mL Hep+EDTA)

All samples were transferred from heparin into EDTA within 2 minutes.

Sample processing order was randomized. Each sample was run in triplicate through an in-house hematology analyzer (Zoetis HM5). Blood smears were also produced, stained with a rapid Romanowski type stain, and evaluated by a board certified clinical pathologist.

Median cat age was 6 years (3-11y), with 17 castrated males and 3 spayed females. Ten specific hematology parameters were analyzed (RBC, HCT, HGB, MVC, WBC, Neut, Mono, Lym, Eos, Plt). For analysis, 1.3mL EDTA was considered the criterion standard.

Correlation between the standard and HCT, HCB, RBC, and MCV was adequate (r >0.9).

Platelet count was the most variable parameter, with some intra-individual samples being highly discordant. Sample size was found to significantly affect platelet count, with very poor correlation between the standard and the 0.5mL samples. MCVs were significantly (though not likely clinically) lower for the 0.5mL Hep+EDTA samples when compared to standard.

The pathologist deemed all slides to be of acceptable quality, with no evidence that the heparin affected morphology. Correlation between the criterion standard and WBCs, neutrophils, and lymphocytes were considered acceptable for all sample types, however monocyte and eosinophil counts had unacceptable variation for all sample types.

Some limitations were present in this study. The analyzers used were in house impedance counters, and results on commercial machines or in-house flow cytometers may be different. Samples sent to laboratories are also often stored for some time, and this may also influence results. The animals used were all healthy cats with relatively normal CBCs, and so the effects of heparinization were not investigated in cats with altered leukocyte counts or morphology.

The authors conclude that the use of smaller samples (0.5mL) and transfer from heparin to EDTA does not significantly influence routine hematology parameters in cats, with the exception of platelets. As feline platelet counts are notoriously unreliable on automated machines and should always be manually verified, this may not be of great significance. Further work in larger numbers of cats and with different machines is needed to confirm these findings. (MRK)

See also:

Flatland B, Freeman KP, Vap LM, et al. ASVCP guidelines: quality assurance for point-of-care testing in veterinary medicine. Vet Clin Path 2013; 42: 405–423.