Establishing an institution-specific therapeutic range for heparin

The relationship between heparin concentration and activated partial thromboplastin time (aPTT) in pooled plasma was compared with that in patient samples to assess the feasibility of using heparin-spiked pooled plasma in determining a therapeutic range for aPTT. Blood samples were taken from 32 patients who had been receiving intravenous unfractionated heparin sodium for more than 24 hours. The samples were stored at -70 degrees C until anti-Xa assay within three months of collection. Pooled normal plasma was spiked with unfractionated heparin sodium to produce nominal anti-Xa concentrations of 0, 0.05, 0.1, 0.2, and 0.5 unit/mL. Heparin concentrations and a aPTT values were measured, and the relationship between the two was determined by linear regression. For the ex vivo samples, the range of aPTT values corresponding to therapeutic heparin concentrations of 0.3-0.7 anti-Xa unit/mL was 64-106 seconds, which corresponds to an aPTT range of 2.3-3.9 times the mean of the normal range (compared with the traditionally defined therapeutic range of 1.5-2.5 times the control value). For the in vitro samples, the aPTT range corresponding to heparin concentrations of 0.3-0.7 unit/mL was 121-256 seconds, which corresponds to an aPTT range of 4.4-9.4 times the mean of the normal range. Each institution should establish a therapeutic aPTT range by calibrating aPTT values against heparin concentrations from blood samples of patients receiving intravenous heparin.