Heparin therapy and its laboratory control

In vitro evaluation of heparin fractions: old vs. new methods

Conventionally, heparin has been evaluated for its anticoagulant effect by various nonspecific, poorly standardized coagulant methods such as the U.S. Pharmacopeial and other global (APTT) tests. However, these assays are relatively insensitive to heparin fractions and fragments, and give varying with the newer modes of heparin administration. It has become necessary, therefore, to develop other methods able to detect a proper therapeutic/prophylactic level and to evaluate a standard potency for these agents. Newer amidolytic assays utilizing synthetic substrates provide for specific anti-factor Xa or anti-factor IIa evaluation. Although these factors are believed to reflect the antithrombotic effect of heparin and its fractions/fragments, conclusive clinical studies are not available at this time. Interactions with non-AT-III-mediated pathways, the contact, kallikrein-kinin, and fibrinolytic system, other coagulant factors, endothelium, and eicosanoid system all can contribute to the overall antithrombotic response of heparin. Some of these actions are not measurable by current in vitro test methods. Amidolytic, immunochemical, and physical assays using various activators to generate endogenous factor Xa, as well as assays for the detection of metabolic or release products of each of these systems, may be more relevant to in vivo conditions. Specific low molecular weight markers such as fibrinopeptide A, which provide the earliest detection of hemostatic activation, can be measured directly, or an in vitro system can be developed to quantitate the relative generation of these markers. These methods can then be modified to assay the in vitro potency of heparin preparations. With proper consideration of reagent specificity and concentration, molecular interactions, reaction time and temperature, ionic type and concentration, reliable accurate methods for a true in vitro representation of in vivo events may be developed. Clinical assays must measure an endogenous response to therapy and not merely an absolute circulating level of drug. However, a standardized pure enzyme system may be ideal to evaluate the potency of these agents. The development of heparin derivatives necessitates a reassessment of currently practiced laboratory technology. Furthermore, well-defined biochemical methods are required for a standardized evaluation of potency and clinical monitoring of these agents.

Comparison of the in vivo hemorrhagic and antithrombotic effects of a low antithrombin-III affinity heparin fraction

Antithrombin III (ATIII) affinity chromatography of commercial grade heparin yields fractions of high and low affinity for ATIII. In vitro, the high affinity fraction accounts for most of the anticoagulant effect while there is evidence that the low affinity material interferes with platelet function. We have studied the relative antithrombotic and hemorrhagic effects of low affinity heparin. The low affinity heparin fraction, specific activity 43 USP units/mg, was compared with standard heparin (150 USP units/mg) in rabbit experimental models. A residual 12.5% by weight of this low affinity heparin showed high affinity for ATIII. Inhibition of thrombosis in a stasis-hypercoagulability model was directly related to the weight (mg) of high affinity material in each of the heparins. In the bleeding model, when similar weights (mg) of high affinity material were infused, significantly more bleeding was demonstrated with the low affinity fraction which contained a 5-fold excess by weight of low affinity material. We have demonstrated that a low affinity heparin depleted of in vitro anticoagulant and in vivo antithrombotic activity significantly contributes to hemorrhage.

Inhibition of aminoglycoside activity in heparin

Measurements of aminoglycosides by an agar disk diffusion assay are inhibited by heparin in a dose-dependent way. When assayed by a homogeneous immunoassay, this was only evident for tobramycin. This indicates that specimens for aminoglycoside measurement should not be obtained in heparinized tubes. When heparin is used clinically as an anticoagulant, the amount in blood does not reach levels that affect the aminoglycoside activity.

Heparin bleeding due to qualitative platelet dysfunction

On two separate occasions, a 26-year-old white woman bled from arterial puncture wounds while receiving heparin for thromboembolic disease. Bleeding time was prolonged after heparin administration at the time that she was ill and bled, and when she was re-challenged 2 years later. Heparin may produce bleeding as a result of a qualitative platelet dysfunction.

Impairment by heparin of primary haemostasis and platelet [14C]5-hydroxytryptamine release

Heparin was administered to 34 normal subjects by intravenous injection (100 mu/kg) and the template bleeding time was significantly increased both 10 min and 120 min following injection. Before heparin the bleeding time was 5-3 +/- 1.0 min (mean +/- 1 SD); 10 min after injection it was 9.8 +/- 5.6 min (P less than 0.001); and 120 min after injection it was 7.2 +/- 3.9 min (P less than 0.001). Increases in the bleeding time were unrelated to changes in platelet count, and independent of heparin's effect on plasma coagulation. In blood drawn 10 min and 120 min following heparin injection, there was significantly less [14C]5-HT released from platelet-rich plasma (PRP) in response to collagen, 0.41 mM epinephrine and 8 micron ADP, although in vitro addition of heparin (0.1 mu/ml, 0.5 mu/ml and 2.5 mu/ml) to baseline PRP of three subjects did not depress [14C]5-HT release. Our experiments suggest that intravenous administration of a therapeutic dose of heparin can cause a significant reversible inpairment of platelet haemostatic properties, possibly by an indirect mechanism.

Management of heparin therapy: Controlled prospective trial

Among 100 consecutive patients receiving heparin in therapeutic dosage, major bleeding occurred in 21, and minor bleeding in 16. Two patients died from bleeding, and two had recurrent pulmonary embolism. Major bleeding occurred in 21% when therapy was regulated with whole-blood clotting time and in 20% when heparin was given without clotting tests. In a subsequent prospective trial patients received heparin by intermittent intravenous injection with or without laboratory control according to the partial thromboplastin time or continuously by intravenous infusion. Recurrent thromboembolism occurred once in each group. Major bleeding was seven times more frequent with intermittent injection than with continuous infusion. Control with the partial thromboplastin time did not prevent major bleeding in patients receiving intermittent injections. With continuous infusion, one-fourth less heparin was required than with intermittent injections. Administration of heparin by continuous infusion appears safer than intermittent injection with or without laboratory control and is no less effective for prevention of thromboembolism.

Inhibitory effect of heparin on gentamicin concentrations in blood

In monitoring gentamicin concentrations in the blood of patients with renal insufficiency, the assayed antibiotic concentration was found to be lower when the sample was drawn as heparinized plasma rather than as serum. This lowering of gentamicin concentrations by heparin was studied further by adding increasing doses of heparin and various amounts of gentamicin to human serum. With a range of 2 to 100 units of heparin per ml, gentamicin concentrations in the serum were lowered by 9 to 14%; with higher heparin concentrations, an even greater and increasing inhibition was noticed, reaching 56% for 1,000 units/ml. This inhibitory effect of heparin on gentamicin was reversible by dilution, indicating that it was not due to degradation or to formation of an inactive chemical complex. Underestimation by the laboratory of gentamicin concentrations in blood is likely to be greatest with capillary tubes, with which the concentration of heparin is especially high. With clinical heparinization, the amount of active heparin in the blood does not exceed 10 units/ml and is for the most part under 3 units/ml; thus, therapeutically significant inhibition of the antibiotic is unlikely in patients receiving anticoagulation.