Platelet Size
Large platelets (megathrombocytes) are generally more active than normal sized platelets. Conversely, the presence of predominantly small platelets (microthrombocytes) in canine blood appears to be one indicator of immune-mediated thrombocytopenia (IMT). Small platelets (mean platelet volume < 5.4 fl) were found in 17 of the 31 IMT cases in study of 68 thrombocytopenia cases (Northern and Tvedten, JAVMA 200: 368-376, 1992).

Platelet Numbers and Size in Citrate vs EDTA Anticoagulated Blood
Platelet numbers and size can vary when blood is collected in EDTA (LTT) vs trisodium citrate (BTT). Platelet counts from BTT are usually lower than those of LTT. Further, platelet counts from stored BTT blood average ~ 30% lower than those of freshly drawn samples, and show platelet clumping to variable degrees. In contrast, minimal platelet clumping occurs in fresh BTT or in fresh or stored LTT.

Platelet Changes During Storage in EDTA Blood
Platelets stored in EDTA (LTT) for up to 72 h remain in a resting state but form platelet-surface associated immunoglobulin G (PSA IgG). These results have important clinical implications for the diagnosis of immune-mediated thromboctopenia , as samples more than 24 h old can give false-positive test PSA IgG test results (Wilkerson and Shuman, Vet Clin Pathol 30:107-113, 2001).

Platelets in Cavalier King Charles Spaniels
Healthy members of this breed often show low circulating platelet counts and can have large platelets that sort outside the platelet "window" of automated platelet counters. These dogs do not require treatment for thrombocytopenia. Typical platelet counts range from 20,000-85,000/µl in clinically normal dogs. Whether the chronic cardiac valvular disease prevalent at a young age in this breed could be a contributing factor is unknown, although no association has been found between the presence of cardiac murmurs and low platelet counts.