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July 2008

Question Q. When you obtain a triglyceride result above 1,000 mg/dL by colorimetric determination and the cholesterol result is below 300 mg/dL, is it correct to report very-low-density lipo­pro­tein, or VLDL, calculation?

A. Estimating VLDL concentration from triglycerides is not valid when the tri­glyc­er­ides exceed 400 mg/dL (4.5 mmol/L). The Friedewald equation, LDL-C = TC – HDL-C – triglycer­ides/5, is used to estimate LDL cholesterol. The term “triglycerides/5” is used to estimate the VLDL-cholesterol concentration. In estimating VLDL from triglycerides, one assumes there are no chylomicrons in the sample and that there are no unusual lipoproteins in the sample that would alter the typical ratio of triglycerides to VLDL particles. It is well document­ed in the literature that the Friedewald equation begins to give less reliable results when the triglycerides exceed 200–300 mg/dL (2.3–3.4 mmol/L), and it is not recommended when the triglycerides are greater than 400 mg/dL (4.5 mmol/L) because the term “triglycerides/5” no longer provides a reliable estimate of the VLDL-C.1-4

References

  1. Warnick GR, Knopp RH, Fitzpatrick V, et al. Estimating low-density lipoprotein cholesterol by the Friedewald equation is adequate for classifying patients on the basis of nationally recommended cutpoints. Clin Chem. 1990;36:15–19.
  2. McNamara JR, Cohn JS, Wilson PW, et al. Calculated values for low-density lipoprotein cholesterol in the assessment of lipid abnormalities and coronary disease risk. Clin Chem. 1990; 36: 36– 42.
  3. Nauck M, Graziani MS, Bruton D, et al. Analytical and clinical performance of a detergent based homogeneous LDL-cholesterol assay: a multicenter evaluation. Clin Chem. 2000;46:506–514.
  4. Tremblay AJ, Morrissette H, Gagné JM, et al. Validation of the Friedewald formula for the determination of low-density lipoprotein cholesterol compared with beta-quantification in a large population. Clin Biochem. 2004;37:785–790

Greg Miller, PhD
Professor of Pathology
Virginia Commonwealth University
Richmond

Question Q. Should unfractionated heparin be monitored by APTT or by an anti-factor Xa method specifically calibrated using unfractionated heparin standard? How would you recommend monitoring direct throm­bin inhibitors?

A. The activated partial thromboplastin time, or APTT, continues to be a frequently used coagulation assay to monitor unfractionated heparin. Laboratories using this assay to monitor heparin should establish a therapeutic range for their institution by correlating APTT results with heparin levels performed by an anti-factor Xa method. The APTT is widely available, inexpensive, and familiar to nurses and physicians who administer weight-based protocols for heparin to patients experiencing thromboembolic events. However, the APTT is not perfect for monitoring heparin. The assay is influenced by coagulation factor deficiencies and circulating coagulation inhibitors such as lupus anticoagulants, and it shows interlaboratory variation in sensitivity to heparin. A heparin level gives a quantitative result that can be compared to a therapeutic range (0.3–0.7 U/mL) and is not affected by other conditions that prolong the APTT. A study by Levine, et al.,1 showed that patients requiring high daily doses of heparin to maintain therapeutic APTT results required less heparin, and experienced less bleeding, when monitored by a heparin level than when monitored by the APTT. A study by Rosborough2 demonstrated fewer monitoring tests and dosage changes when monitoring with anti-factor Xa than with the APTT. These findings have led to an increased interest in using heparin levels for monitoring heparin, but some issues must be considered. A heparin level may be more expensive to perform than the APTT and may not be offered in smaller laboratories.3 The decision of whether to use the APTT or a heparin level to monitor unfractionated heparin is currently left to each institution and depends on patient population and laboratory resources. Laboratories changing from the APTT to heparin level for monitoring unfractionated heparin will want to work with their clinicians, pharmacists, and nurses to implement dosing algorithms that incorporate heparin levels. Additionally, heparin protocols for individual patients based on sex, age, height, and weight are available at www.abbottnorthwestern.com/medicationprotocols.4

These protocols should include monitoring the platelet count for heparin-induced thrombocytopenia/thrombosis, whether the APTT or heparin level is used. Some institutions may continue to use the APTT to monitor routine heparin therapy, but they may also offer a heparin level for patients with clinical conditions that are known to: 1) prolong the baseline APTT, 2) cause decreased sensitivity of the APTT to heparin (that is, elevated factor VIII or fibrinogen activity level), or 3) confer true heparin resistance (that is, antithrombin deficiency, increased plasma binding/clearance of heparin).3

Direct thrombin inhibitors, or DTIs, differ from heparin in that they inactivate thrombin directly rather than enhance the effect of antithrombin. Argatroban, lepirudin, and bivalirudin, which are commonly used DTIs, usually do not require laboratory monitoring. When monitoring is desired, the APTT is often used. Manufacturers’ guidelines are to maintain the APTT at one and a half to three times the patient’s baseline, not to exceed 100 seconds for argatroban, and one and a half to two and a half times the median of the laboratory’s APTT reference range for lepirudin. The APTT has the same limitations in this clinical scenario as when it is used to monitor unfractionated heparin, and in addition is nonlinear with higher concentrations of the DTI.5 If monitoring DTIs is necessary, using tests most appropriate to the DTI may be the best approach. Tests other than the APTT include the ecarin clotting time, ELISA for lepirudin, argatroban level using a clotting anti-factor IIa, a chromogenic assay for anti-factor IIa, and activated clotting time as a point-of-care method. In the future, the thrombin generation assays may be used to monitor DTI therapy. A recent study by Love, et al.,6 described experience with monitoring DTIs using a plasma-diluted thrombin time, which appeared to be an alternative to the APTT. Knowing the DTI most frequently used by clinicians in your facility can help guide the laboratory in choosing the best tests to offer. Protocols for monitoring DTI should be developed in cooperation with other members of the health care team as recommended for unfractionated heparin monitoring.

References

  1. Levine MN, Hirsh J, Gent M, et al. A randomized trial comparing activated thromboplastin time with heparin assay in patients with acute venous thromboembolism requiring large daily doses of heparin. Arch Intern Med. 1994;154: 49–56.
  2. Rosborough TK. Monitoring unfractionated heparin therapy with antifactor Xa activity results in fewer monitoring tests and dosage changes than monitoring with the activated partial thromboplastin time. Pharmacotherapy. 1999;19:760–766.
  3. Olson JD, Arkin CF, Brandt JT, et al. College of American Pathologists conference XXXI on laboratory monitoring of anticoagulant therapy: laboratory monitoring of unfractionated heparin therapy. Arch Pathol Lab Med. 1998:782–798.
  4. Rosborough TK, Shepherd MF. Achieving target antifactor Xa activity with a heparin protocol based on sex, age, height, and weight. Pharmacotherapy. 2004;24: 713–719.
  5. Gray E, Harenberg J. Collaborative study on monitoring methods to determine direct thrombin inhibitors lepirudin and argatroban. J Thromb Haemost. 2005;3: 2096–2097.
  6. Love JE, Ferrell C, Chandler WL. Monitoring direct thrombin inhibitors with a plasma diluted thrombin time. Thromb Haemost. 2007;98:234–242.

Sandra C. Hollensead, MD
Department of Pathology and Laboratory Medicine
University of Louisville
School of Medicine
Louisville, Ky.
Member, CAP Coagulation Resource Committee