A low volume specimen container suitable for monitoring the aPTT of heparinized patients

Abstract

Differences in the activated partial thromboplastin time (aPTT) were shown when blood taken from patients receiving intravenous heparin therapy was collected into 5 ml and 1 ml citrate containers. Mean aPTTs were 27% shorter with the plasma from the 1 ml citrate containers (n = 23). These results were paralleled by a 37% reduction in the mean heparin concentration (n = 11) and a 77% increase in the mean platelet factor 4 (PF4) concentration (n = 7). This phenomenon is due to increased platelet activation and subsequent increased heparin neutralization in the 1 ml citrate container. In an attempt to overcome this, the citrate was removed from a 1 ml container and replaced with a buffered tri-sodium citrate solution containing theophylline, adenosine and dipyridamole anticoagulant (CTAD). Blood from heparinized patients taken into both 5 ml citrate and 1 ml CTAD showed a correction of the shortening artefact in the low volume container. The mean aPTT of plasmas from the 1 ml CTAD container showed an increase of 10% compared with the 5 ml citrate. There was no significant difference in the mean heparin or PF4 concentrations of blood taken into either container. The 1 ml CTAD tube described is a suitable collection container for monitoring heparin in neonates or patients who are difficult to venepuncture and overcomes the neutralization of heparin in part filled low volume containers.