Standardization of APTT reagents for heparin therapy monitoring: urgent or fading priority?

Tremendous improvements have been made in the outcomes of patients with acute venous thromboembolic events (VTE) since Murray et al. successfully administered heparin to postsurgery patients in 1937 [1]. Advances include accurate methods for diagnosis of deep vein thromboses (DVT) and pulmonary emboli (PE); effective regimens for initial and long-term anticoagulation; and wide acceptance of a system for standardization of thromboplastin sensitivities to the anticoagulant effects of warfarin (International Normalized Ratio; INR). In this issue of Clinical Chemistry, van den Besselaar et al. [2] address an unresolved aspect of anticoagulation therapy: standardization of activated partial thromboplastin time (APTT) reagents for monitoring heparin therapy. Heparin is a glycosaminoglycan extracted from mast cells of porcine intestinal mucosa or bovine lung. It is composed of long chains of alternating d-glucuronic acid and N-acetyl-d-glucosamine sugar residues, which undergo a series of chemical modifications, primarily sulfation, leading to unique pentasaccharide sequences that serve as a binding site for antithrombin III (AT III) [3]. The anticoagulant effect of heparin is mediated through this interaction, which markedly accelerates the rate of AT III inhibition of thrombin (factor IIa) and factor Xa. Heparin polysaccharides are heterogeneous in length and anticoagulation activity and range in mass from 5000 to 30 000 kDa. Commercial preparations are calibrated in USP units, 1 unit being defined as the quantity that prevents 1.0 mL of citrated sheep plasma from clotting for 1 h after the addition of 0.2 mL of 10 g/L CaCl2 [3]. Low-molecular-weight heparins (LMWH) are produced from unfractionated heparin to yield smaller polysaccharides with average molecular masses of 4000– 5000 kDa. These shorter molecules lose the ability to accelerate AT III inhibition of thrombin but retain the ability to catalyze factor Xa inhibition. Decreased in vivo protein binding improves LMWH bioavailability and leads to predictable anticoagulant response [4]. In 1960, Barrett and Jordan conducted the first prospective study of anticoagulation therapy, randomizing patients with pulmonary emboli to treatment with 10 000 USP units of heparin intravenous bolus every 6 h for 6 doses and an oral anticoagulant for 2 weeks, or no therapy [5]. None of the 16 patients who received heparin died of a recurrent PE, compared with 5 of 19 PE-related deaths in the control group. However, one patient who received heparin died of complications from a bleeding duodenal ulcer. Subsequently, investigators realized that the anticoagulant response to a fixed dose of heparin was variable in acutely ill patients. In an effort to administer a sufficient amount of heparin to prevent recurrent thromboembolic complications yet low enough to carry an acceptable risk of hemorrhage, therapeutic ranges based on laboratory monitoring of the anticoagulant effect of heparin were developed. A combination of clinical experience [6] and results from animal models of experimental venous stasis and thrombosis [7, 8] supported a therapeutic target of doubling the whole-blood clotting time or prolongation of the APTT to 1.5 to 2.5 times the laboratory mean control. The APTT quickly became the accepted test for monitoring heparin activity because of its convenience, precision, wide availability, and adaptability to automated instruments. A correlation between plasma heparin activity, APTT prolongation, and thrombus growth, established in 1977 by Chui et al., has become a cornerstone of heparin therapy [9]. They measured fibrin accumulation onto a thrombus in an isolated segment of a rabbit jugular vein. Minimal additional accumulation of fibrin occurred during heparin infusion rates that produced plasma heparin activities between 0.2 and 0.4 USP unit/mL (based on protamine titration) and APTTs of 1.5 to 2.5 times baseline. Higher heparin concentrations produced minor additional reductions in fibrin accretion and longer bleeding times from jugular vein punctures. Over the last two decades, clinical investigators have conducted a series of prospective, randomized clinical trials comparing different anticoagulation strategies for the management of patients with acute VTEs. Measured outcomes included recurrent thromboembolic events and bleeding. These studies have convincingly shown that: 1. Initial treatment with an oral anticoagulant alone leads to more recurrent VTEs than does combination therapy with heparin [10]. 2. Recurrence rates are higher when subtherapeutic APTTs persist for .24–48 h after initiation of heparin [11]. 3. Five days of therapeutic continuous intravenous heparin therapy is as effective as 10 days when warfarin is begun on day 1 rather than day 5 [12]. 4. Nomograms for heparin dosing (intravenous bolus, then continuous infusion) increase the proportion of patients with APTTs above a minimal therapeutic threshold and improve outcome (lower recurrence rates) [13]. 5. Weight-based dosing of SQ LMWH without laboratory monitoring is as efficacious as continuous infusion of unfractionated heparin with APTT therapeutic monitoring [14]. The efficacy rates of continuous heparin infusions were similar in these clinical trials (VTE recurrence ;6% [4], bleeding complications ,5% [15]). APTT therapeutic ranges were usually equivalent to plasma heparin activities of 0.2–0.4 USP units/mL by protamine titration, and average initial rates of heparin infusion were $30 000 units/24 h [4]. However, reliance on a 1.5 to 2.5 times control APTT therapeutic range to guide heparin dosing in routine clinical practice is fraught with many sources of variability, as was the historical 2–2.5 times control PT target for Editorial