Heparin and the activated partial thromboplastin time--a difference between the in-vitro and in-vivo effects and implications for the therapeutic range.

The in-vivo sensitivity to heparin of activated partial thromboplastin time (aPTT) reagents is different from the in-vitro sensitivity. Equivalent therapeutic ranges for reagents of very different sensitivities may be determined from the regression line of aPTT against whole blood clotting time. The commonly used ratio of 1.5 to 2.5 times the normal control value is appropriate for some reagents, but for the other reagents a higher or lower range is appropriate. A suitable aPTT reagent for heparin control should be sensitive both to a threshold effect of heparin and to increasing dosage. Some reagents discriminate poorly at the threshold level.