Effect of heparin on whole blood activated partial thromboplastin time using a portable, whole blood coagulation monitor.

OBJECTIVES

To evaluate the responsiveness of whole blood activated partial thromboplastin time (aPTT) to varying heparin doses in vitro and to examine the ex vivo relationship of whole blood aPTT to plasma heparin concentration.

DESIGN

Prospective, controlled laboratory study.

SETTING

Surgical suites and laboratory at a tertiary center.

PATIENTS

Surgical patients and volunteers at a tertiary center were eligible for inclusion in this study. In vitro evaluation was performed using specimens obtained from each of five, healthy volunteers. Ex vivo evaluation was performed using specimens obtained from 30 cardiac surgical patients before and after systemic administration of heparin for extracorporeal circulation.

INTERVENTIONS

Blood specimens were obtained from volunteers and added to syringes containing varying amounts of unfractionated porcine heparin for in vitro evaluation. For ex vivo evaluation, blood specimens were obtained from patients before and after systemic administration of 20 U/kg of heparin.

MEASUREMENTS AND MAIN RESULTS

For the in vitro evaluation, specimens were divided into two aliquots after mixing with varying amounts of unfractionated porcine heparin. One aliquot was used to measure whole blood aPTT using a whole blood coagulation monitor immediately after blood collection and 3 mins later, and a second aliquot was used to determine plasma aPTT with a conventional, laboratory-based assay. Linear regression analysis demonstrated a high correlation (r = .94; r2 = .88) between aPTT assay systems and bias analysis demonstrated a mean aPTT measurement difference of 1.6 secs with +/- 2 SD limits of -15 to +18.2 secs. As indicated by comparable regression slopes, the in vitro aPTT responsiveness to increasing heparin concentration was similar with the two assay systems among individual subjects. Whole blood aPTT measurements after 3 mins of blood specimen storage were similar to immediate measurements. For ex vivo evaluation, blood specimens obtained from patients before and after systemic administration of heparin were divided into two aliquots. One aliquot was used to measure whole blood aPTT in duplicate and a second aliquot was used to measure plasma heparin concentration with an antifactor X active chromogenic assay. A high correlation (r = .89; r2 = .79) between whole blood aPTT and plasma heparin concentration was observed.

CONCLUSIONS

Heparin responsiveness of whole blood aPTT, measured with a portable whole blood coagulation monitor, is similar to that of conventional laboratory aPTT over a clinically relevant range of heparin concentrations in vitro and ex vivo. On-site whole blood aPTT measurements should be useful in clinical situations requiring rapid aPTT results.