Variable response of activated partial thromboplastin time to heparin therapy during hemodialysis

The consistency of the anticoagulant effect of intravenously administered heparin was studied. The activated partial thromboplastin time (APTT) was measured for six patients hourly during three consecutive hemodialysis sessions each. Cephaloplastin was the plasma-activating agent. The time required to form a clot was measured by a light-sensitive electronic timer and confirmed within +/- 5% by the tilt tube method. Results are reported in second relative to units of heparin given to patients per kilogram body weight. The range of APTT's measured 55 minutes after each heparin dose greatly exceeded the range of technical variability of the assay method. The probably mechanisms and consequences for this variability after a constant heparin dose are discussed. The anticoagulation effect of heparin during hemodialysis in an otherwise stable clinical situation is not constant. The risks of having too much or too little anticoagulation are not eliminated by having determined a therapeutic heparin dose during one dialysis rung.