ACTIVATED PARTIAL THROMBOPLASTIN TIME APPENDIX

Hemolysis significantly shortens the activated partial thromboplastin time in normal but not in abnormal persons.¹ The APTT may be normal in persons with mild hereditary bleeding disorders. About 30% of normal concentration of factors V, VIII, IX, X, XI, and XII will maintain a rate of thrombin formation sufficient to produce a normal APTT. Prolongation of the APTT clotting time occurs if the concentration of any of the above single clotting factors falls below this level. Fibrinogen level, if <80 mg/dL, may result in an abnormal APTT. If Fletcher or Fitzgerald factors are <5% of normal, the APTT may be abnormal.² A prolonged APTT can be caused by inherited factor deficiency (I, II, V, VIII-XII), Fletcher or Fitzgerald, Coumadin® type therapy, liver disease, circulating anticoagulant (heparin, lupus anticoagulant, fibrin breakdown products), specific factor inhibitor (rheumatoid arthritis, penicillin reaction, occasional hemophiliacs), or intravascular coagulation.

The results of the College of American Pathologists surveys indicate that the source and type of heparinized specimen is important to consider when interpreting APTT test results.³ Different reagent/instrument combinations effect the APTT response to heparin. Sensitivity is most influenced by the APTT reagent used while precision is most effected by the instrument utilized.

Control of heparin therapy: The control of heparin anticoagulant therapy is complex, controversial, and problematic. Recommendations for dosage and laboratory monitoring will be followed by a consideration of factors that may be responsible for "failures" (failure to achieve anticoagulated state, and/or excessive anticoagulation).

Heparin: Heparin is an acidic mucopolysaccharide found in mast cells and basophils, has a strong negative charge, has a circulating half-life of only a few hours, and inhibits all of the active serine proteases (IIa, Xa, IXa, XIa, and XIIa). Heparin is stable for 24 hours in a 5% dextrose solution.⁴ Low dose heparin activity relates to inactivation of Xa and possibly to cell surface repulsion effects. On the basis of minimum dose of heparin effective in producing comparable prolongations of clotting time, the whole blood PTT, APTT, whole blood clotting time, and PTT are about equivalent in measuring response to heparin.⁵ While some inactivation of heparin may occur in the liver (through action of heparinase), elimination is largely by the kidney so that heparin must be used cautiously in cases of impaired glomerular filtration.⁶

Administration, dosage: Heparin is best administered intravenously, intermittently, or better as continuous infusion^7,8,9 in a dosage of 400-500 units/kg body weight/day divided into every 6-hour dosage (so that 100-125 units/kg body weight is given each 6 hours). Laboratory monitoring can be accomplished using the Lee-White clotting time, APTT, or ACT. Dosage is adjusted to maintain the coagulation test result at about two times the control or "normal" level.

There are three levels of heparin therapy: low dose, moderate dose, and large dose. Before proceeding, one must decide on the level to be employed.⁷ Low dose refers to small amounts of heparin (10,000-20,000 units/day) given subcutaneously and useful in the prophylaxis against venous thrombosis (selected patients). Measurable change in activated partial thromboplastin time (APTT) does not usually occur. Moderate dose implies full anticoagulation, requires 20,000-60,000 units/day, the APTT is adjusted to 1¹/₂-2 times the baseline, and the regimen is applied to patients without active thromboembolic disease. Large dose heparin therapy utilizes dosage levels of 60,000-100,000 units/day during the first 24-48 hours and then reverting to 30,000-45,000 units/day. Large dose therapy is for patients with active thromboembolic disease.

Heparin should not be given intramuscularly.

Complications: Not all individuals respond ideally or predictably to heparin. A voluminous literature deals with these exceptions and one must conclude that there is no short cut to adequate and safe heparin anticoagulation. One must understand and investigate the causes and management of aberrant cases.

Drugs which antagonize the action of heparin include streptomycin, erythromycin, gentamicin, chlorpromazine, ascorbic acid, antihistamines, and digitalis.

Heparin should be given intravenously or subcutaneously but not intramuscularly due to the high risk of hematoma formation. Anaphylaxis and erythematous reactions may occur with the use of heparin in some individuals.

Untoward effects of heparin include (1) development of osteoporosis with fractures (doses of 20,000 units/day over 6 months) for which a calcium intake of 1 g or more per day may be protective, (2) anti-inflammatory effect, and (3) inhibition of antidiuretic effect resulting in diuresis. Unusual reactions to heparin include anaphylaxis and erythematous reactions (species specific), alopecia, urticaria, headache, and bronchospasm.

An especially important complication of high dose heparin therapy is the development of thrombocytopenia which relates to heparin-induced aggregation. Because of the dangers attendant to a minority of heparin anticoagulated individuals, pretherapeutic laboratory evaluation has been recommended.¹⁰ Abnormalities in platelet count, whole blood clotting time, activated PTT, or antithrombin III may indicate that the patient has a predisposition to an unusual heparin response. Hussey et al have emphasized the significant morbidity (including amputation of extremities) and mortality associated with heparin induced platelet aggregation resulting in new thrombosis and thrombocytopenia.¹¹ This phenomenon appears to have immune etiology and occurs in the presence of either an IgG or IgM heparin dependent platelet aggregating antibody.^12,13 These patients have a measurable abnormal response to ADP, heparin and ADP, and heparin alone in the platelet aggregation procedure when thrombocytopenia is present.

The progressive thromboembolic syndrome appears to be always associated with thrombocytopenia. Monitoring of the heparinized patient has been recommended and includes daily physical examination for evidence of further thrombosis and periodic platelet counts, the need for which is determined clinically. Evidence of new thrombosis or decrease in platelet count to <100,000/mm³ should be further investigated with aggregation studies to see if an abnormal response to platelet aggregation is present.

The platelet abnormality quickly reverses when heparin is discontinued. Added beneficial therapy includes antiaggregating agents such as dextran (Rheomacrodex®) I.V., 25 mL/hour; acetylsalicylic acid; dipyridamole; and Coumadin®. Iloprost® has been utilized to prevent heparin-induced thrombocytopenia during open heart surgery.^14,15

The table summarizes the relation of three coagulation tests that have been and can be utilized to adjust the heparin anticoagulant effect.

Heparin Anticoagulant Effect

Early studies^16,17 suggested that thrombi do not propagate when the heparin level is such as to prolong the Lee and White (L&W) clotting time to twice that of a normal control. The level of heparin required or desirable, however, varies on an individual basis depending on the severity of the thrombotic process, the potential bleeding risk, variations in the heparin preparation - varying polymer length, sulfonation of polymers, medications that inactivate or inhibit heparin (antihistamines, digitalis, nicotine, penicillin, tetracyclines, phenothiazines, and protamine), poor coordination in timing the dose and collection of specimens, antithrombin III level, platelet factor IV level, and fibrinogen level.¹⁸ If the latter factors are in normal range, heparin level of 0.3 units/mL of plasma is required to result in a L&W time of twice the normal control. Heparin level of 0.6 units/mL (three times normal L&W clotting time) may result in clinical bleeding. Therapeutic range is usually attained with heparin level of 0.2-0.4 units/mL. Heparin dose of 100-125 units/kg body weight given at 6 hourly intervals bolus I.V. or S.C. or 400-500 units/kg body weight/day as constant infusion will usually provide effective anticoagulation. Constant drip by pump infusion devices is probably most effective, while some have found the constant I.V. drip method to be cumbersome, difficult to control and to be without proven therapeutic superiority.^10,11 With intermittent pulse dosage, however, heparin effect must be monitored so as to provide high level anticoagulant action without accumulation with subsequent doses.

Monitoring: A bewildering number of coagulation tests and protocols have been recommended for monitoring heparin effect. The Lee and White clotting time, activated clotting time (ACT), and the activated plasma thromboplastin time (APTT) are currently used.

Additional Information: The ACT has been favored during cardiovascular operations to monitor heparin dosage and neutralization. The ACT, however, assays overall coagulation activity such that prolonged values may not be exclusively the result of heparin. There is a risk, then, in giving protamine sulfate (heparin antagonist).¹⁹ When the protamine sulfate concentration exceeds that of heparin, it begins to act as an anticoagulant with resultant at least potential lack of specificity. This can be identified by showing correction of the prolonged ACT by addition of protamine sulfate in vitro to the blood in question.¹⁹ Some have held that laboratory control of heparin use is unnecessary.²⁰ They have found that the incidence of major bleeding complications during heparin anticoagulation is essentially the same when therapy is regulated with the whole blood clotting time (WBCT) as when heparin is given without clotting tests.²¹

The BaSon test (modification of whole blood PTT - Harem) was done at the bedside and was found to correlate with the WBCT (Lee and White).^22,23,24

A significantly larger amount of heparin is required for effective anticoagulation in the presence of active thromboembolic disease. It has been suggested that the heparin dosage required to maintain a target APTT of 1.5-2.5 times the patient's baseline has diagnostic importance and can contribute to an understanding of whether or not thromboembolic disease is present.⁷

Simultaneous monitoring of APTT, WBCT, and the thrombin clotting time appears to have identified a population of patients in which a significant increase (more than double) in the level of factor VIII activity occurs.²⁵ This results in a misleading shortening of the APTT. To detect this situation, a combination of APTT and a thrombin clotting time heparin assay (TCT) has been recommended.²⁵ Discrepancy between a normal APTT and a prolonged TCT may indicate presence of antithrombin III deficiency. With dysfibrinogenemia the TCT would be especially prolonged.

The APTT may be excessively sensitive and while many studies show correlation with the whole blood clotting time, the series usually lack cases with prolonged APTT or the data show poor correlation with high levels of anticoagulation.²⁶

The Lee and White clotting time (WBCT) provides useable results but may require 30-40 minutes to achieve endpoint (clot formation) at high heparin levels. The microsilicate activated clotting time (ACT) obviates some of the difficulties with the WBCT, providing shorter clotting times. Kurec AS et al have reported²⁷ their experience comparing WBCT, ACT, and APTT in monitoring heparin anticoagulation. They have found that the ACT correlates best with the APTT and is generally most useable.

Low molecular weight heparin (LMWH): Standard heparin is composed of sulfated mucopolysaccharides, heterogeneous fragments of different molecular weights. LMWH is less heterogeneous, consists of fragments with high and low affinity for AT III but LMWH use is associated with decreased antithrombin activity while anti-Xa activity is largely preserved. There is evidence that LMWH is able to protect against thromboembolic events while the risk of hemorrhage is reduced.²⁸ Conventional tests used in monitoring of standard heparin treatment (eg, APTT, TT, ACT) are not affected by LMWH at the doses usually employed. Laboratory control of the use of LMWH is therefore not applicable. Heparin-induced thrombocytopenia is usually seen, however, some 7-12 days after LMWH therapy begins so that periodic platelet counts are recommended.²⁸

In recent years, increased emphasis has been placed on prevention of bleeding as a complication of anticoagulation, one of the goals of laboratory monitoring. To this end, use of a bleeding risk index for prospective evaluation has been developed and found to provide a valid estimate of the probability of major bleeding during anticoagulation.²⁹

Patients with a prolonged APTT not corrected by mixing with normal plasma but with no family or clinical/surgical history of bleeding are (when evaluated) most commonly due to a phospholipid (lupus type) anticoagulant. College of American Pathologists surveys (1986 and 1987) found significant difference in the sensitivity of different APTT reagents to the presence of lupus anticoagulants. The difference in reagent responsiveness can affect the apparent factor activity and also the dilutional effect on mixing patient with normal plasma samples, thus impairing the ability to differentiate a lupus anticoagulant from a specific factor inhibitor.³⁰

Appropriateness of prothrombin and partial thromboplastin time testing on the medical service of a teaching hospital (ordering patterns in relation to clinical indications) concludes that these tests are overutilized (at least 70% were not clinically indicated).^31,32

Over 50% of cases of Noonan's syndrome (congenital heart disease, short stature, and dysmorphic facies) also have abnormal bleeding. Prolonged APTT was found in 40% of patients with the syndrome. A variety of specific individual and combined deficiencies were identified.³³

Footnotes

1. Garton S and Larsen AE, "Effect of Hemolysis on the Partial Thromboplastin Time,"Am J Med Technol, 1972, 38(10):408-10.

2. Penner J, "Activated Partial Thromboplastin Time,"Blood Coagulation Manual, University of Michigan Medical School, 1979, 70.

3. Gawoski JM, Arkin CF, Bovill T, et al, "The Effects of Heparin on the Activated Partial Thromboplastin Time of the College of American Pathologists Survey Specimens. Responsiveness, Precision, and Sample Effects,"Arch Pathol Lab Med, 1987, 111(9):785-90.

4. Stock SL and Warner N, "Heparin in Acid Solutions,"Br Med J, 1971, 3(769):307.

5. Estes JW, "Kinetics of the Anticoagulant Effect of Heparin,"JAMA, 1970, 212(9):1492-5.

6. Gurewich V, Thomas DP, and Stuart RK, "Some Guidelines for Heparin Therapy of Venous Thromboembolic Disease,"JAMA, 1967, 199(2):116-8.

7. White TM, Bernene JL, and Marino AM, "Continuous Heparin Infusion Requirements. Diagnostic and Therapeutic Implications,"JAMA, 1979, 241(25):2717-20.

8. Salzman EW, Deykin D, Shapiro RM, et al, "Management of Heparin Therapy: Controlled Prospective Trial,"N Engl J Med, 1975, 292(20):1046-50.

9. Glazier RL and Crowell EB, "Randomized Prospective Trial of Continuous vs Intermittent Heparin Therapy,"JAMA, 1976, 236(12):1365-7.

10. Forman WB, "The Risks of Heparin Therapy,"Hosp Formulary, 1978, 779-84.

11. Hussey CV, Bernhard VM, McLean MR, et al, "Heparin Induced Platelet Aggregation: In Vitro Confirmation of Thrombotic Complications Associated With Heparin Therapy,"Ann Clin Lab Sci, 1979, 9(6):487-93.

12. Trowbridge AA, Caraveo J, Green JB, et al, "Heparin-Related Immune Thrombocytopenia. Studies of Antibody-Heparin Specificity,"Am J Med, 1978, 65(2):277-83.

13. Wahl TO, Lipschitz DA, and Stechschulte DJ, "Thrombocytopenia Associated With Antiheparin Antibody,"JAMA, 1978, 240(23):2560-2.

14. Addonizio VP Jr, Fisher CA, Kappa JR, et al, "Prevention of Heparin-Induced Thrombocytopenia During Open Heart Surgery With Iloprost® (ZK36374),"Surgery, 1987, 102(5):796-807.

15. Sobel M, Adelman B, Szentpetery S, et al, "Surgical Management of Heparin-Associated Thrombocytopenia. Strategies in the Treatment of Venous and Arterial Thromboembolism,"J Vasc Surg, 1988, 8(4):395-401.

16. Wessler S and Morris CE, "Studies in Intravascular Coagulation: IV. The Effect of Heparin and Dicumarol on Serum Induced Venous Thrombosis,"Circulation, 1955, 12:553-6.

17. Carey LC and Williams RD, "Comparative Effects of Dicumarol, Tromexan, and Heparin on Thrombus Propagation,"Ann Surg, 1960, 152:919-22.

18. Soloway HB, "Inappropriate Response to Heparin Therapy,"Diag Med, 1979, 2:31-3.

19. Roth JA, Cakingnan RA, and Scott CR, "Use of Activated Coagulation Time to Monitor Heparin During Cardiac Surgery,"Am Thorac Surg, 1979, 28(1):69-72.

20. Bauer G, "Clinical Experiences of a Surgeon in the Use of Heparin,"Am J Cardiol, 1964, 14:29-35.

21. Salzman EW, Deykin D, Shapiro RM, et al, "Management of Heparin Therapy: Controlled Prospective Trial,"N Engl J Med, 1975, 292(20):1046-50.

22. Baden JP, Sonnenfield M, Ferlic RM, et al, "The BaSon Test: A Rapid Bedside Test for Control of Heparin Therapy,"Surg Forum, 1971, 22:172-4.

23. Blakely JA, "A Rapid Bedside Method for the Control of Heparin Therapy,"Can Med Assoc J, 1968, 99(22):1072-6.

24. Sonnenfield M and Baden JP, "The BaSon Test: A Rapid Bedside Technique for the Control of Heparin Therapy,"Am J Med Technol, 1972, 38:311.

25. Glynn MF, "Heparin Monitoring and Thrombosis,"Am J Clin Pathol, 1979, 71(4):397-400.

26. Ray PK and Harper TA, "Comparison of Activated Recalcification and Partial Thromboplastin Tests as Controls of Heparin Therapy,"J Lab Clin Med, 1971, 77(6):901-7.

27. Kurec AS, Morris MW, and Davey FR, "Clotting, Activated Partial Thromboplastin and Coagulation Times in Monitoring Heparin Therapy,"Ann Clin Lab Sci, 1979, 9(6):494-500.

28. Samama M, "Low Molecular Weight Heparin,"ASCP Check Sample®, Chicago, IL: American Society of Clinical Pathologists, 1987.

29. Landefeld CS, McGuire E 3d, and Rosenblatt MW, "A Bleeding Risk Index for Estimating the Probability of Major Bleeding in Hospitalized Patients Starting Anticoagulant Therapy,"Am J Med, 1990, 89(5):569-78.

30. Brandt JT, Triplett DA, Rock WA, et al, "Effect of Lupus Anticoagulants on the Activated Partial Thromboplastin Time. Results of the College of American Pathologists Survey Program,"Arch Pathol Lab Med, 1991, 115(2):109-14.

31. Erban SB, Kinman SL, and Schwartz JS, "Routine Use of the Prothrombin and Partial Thromboplastin Times,"JAMA, 1989, 262(17):2428-32.

32. Janvier G, Winnock S, and Freyburger G, "Value of the Activated Partial Thromboplastin Time for Preoperative Detection of Coagulation Disorders Not Revealed by a Specific Questionnaire,"Anesthesiology, 1991, 75(5):920-1.

33. Sharland M, Patton MA, Talbot S, et al, "Coagulation-Factor Deficiencies and Abnormal Bleeding in Noonan's Syndrome,"Lancet, 1992, 339(8784):19-21.

References

Andrew M, Paes B, Milner R, et al, "Development of the Human Coagulation System in the Healthy Premature Infant,"Blood, 1988, 72(8):1651-7.

Andrew M, Vegh P, Johnston M, et al, "Maturation of the Hemostatic System During Childhood,"Blood, 1992, 80(8):1998-2005.

D'Angelo A, Seveso MP, D'Angelo SV, et al, "Effect of Clot-Detection Methods and Reagents on Activated Partial Thromboplastin Time (APTT). Implications in Heparin Monitoring by APTT,"Am J Clin Pathol, 1990, 94(3):297-306.

Dhami MS and Bona RD, "Using Anticoagulants Safely. Guidelines for Therapeutic and Prophylactic Regimens,"Postgrad Med, 1991, 90(1):121-2, 127-32.

Middleton AL and Oakley E, "Activated Partial Thromboplastin Time (APTT): Review of Methods,"ASCP Check Sample®, Chicago, IL: American Society of Clinical Pathologists, 1987.

Turpie AG, Robinson JG, and Doyle DJ, "Comparison of High-Dose With Low-Dose Subcutaneous Heparin to Prevent Left Ventricular Mural Thrombosis in Patients With Acute Transmural Anterior Myocardial Infarction,"N Engl J Med, 1989, 320(6):352-7.