Platelet numbers and size can vary when blood is collected in either EDTA or trisodium citrate tubes. The platelet count also can vary between freshly collected and stored blood samples. To assess these variables, blood was drawn daily from 10 dogs into EDTA (LTT) and trisodium citrate (BTT) tubes. The fresh blood was analyzed immediately by automated and slide techniques, and after storing the blood at 4-8° C for 24 hours.

Platelet counts (repeated 5 times on each sample) from BTT, were lower than those of the LTT tubes. These differences were striking when results of the fresh and stored blood samples were compared. Platelet counts from stored BTT blood averaged 30% lower than those of the freshly drawn samples. BTT slide smears from stored blood showed platelet clumping to variable degrees. However, only 1 of the 10 slide smears showed platelet clumping in the fresh BTT, or fresh or stored LTT tubes.

This study confirms that platelet counts are unreliable in blood stored 24 hours (or longer), especially when it is collected in citrate anticoagulant (BTT). Based on these findings, platelet counts should be verified on LTT rather than BTT tubes. For urgent situations, if an EDTA (LTT) tube is unavailable, a stored BTT tube can be used to repeat the platelet count. However, the final count will be about 30% lower (i.e. multiply count obtained by 1.6 to obtain the estimated original number).