Baker et al1 rightly point out that for any heparin concentration, a large variation in activated partial thromboplastin time (aPTT) response is expected, and that for any aPTT, an equally variable heparin concentration is expected. We have previously shown this to be true.2 However, I disagree with their premise that "Patient outcomes appear to correlate with heparin blood levels." To date, no study has adequately shown that regulating heparin concentration is superior to monitoring patients using aPTT. In fact, all major trials for deep venous thrombosis or acute coronary syndromes have used aPTT as the marker of anticoagulant response. Several studies have retrospectively looked at heparin concentration and one, as mentioned below, has used animal data to evaluate heparin concentration. Before accepting that heparin concentration is superior to an established functional end point (aPTT), I believe we need to critically review the foundation of this premise.
Mungall D. Activated Partial Thromboplastin Time vs Heparin Concentration. Arch Intern Med. 1998;158(11):1273-1275. doi: