Effect of Sample Volume on Coagulation Tests

Abstract

This study was undertaken to determine an acceptable range of specimen volumes for routine coagulation tests, that is, the prothrombin time (PT) and activated partial thromboplastin time (APTT) tests. Samples were obtained from healthy volunteers, patients who were receiving warfarin (Coumadin), and patients who were receiving heparin therapy. Specimen volumes, from as little as 3.0 mL to as much as 6.0 mL, were collected from each patient into blue-stoppered evacuated (Vacutainer1) tubes that contained 0.5 mL of 3.8% buffered sodium citrate. Results ofPT and APTT tests from the above specimens were compared with 4.5-mL samples from the same patients that were collected at the same time. Test results in each category of sample volumes, 3.0,3.5,5.5, and 6.0 mL, were compared with the corresponding 4.5-mL sample size. The only statistically and clinically significant difference in test results was in the 3.0-mL sample sizes from healthy volunteers who were tested for APTTs. Although a 4-second difference in the APTT, seen in the 3.5-mL sample size from healthy volunteers, was not statistically significant, this difference may be exaggerated in patients with a higher than normal hematocrit.We can, therefore, recommend that tolerance limits for sample size for commercially available blue-stoppered evacuated tubes that contain 0.5 mL of 3.8% sodium citrate to be at least between 3.5 and 6.0 mL from patients with a normal or lower hematocrit.