The influence of free hemoglobin and bilirubin on heparin monitoring by activated partial thromboplastin time and anti-Xa assay

Arch Pathol Lab Med. 2014 Nov;138(11):1503-6. doi: 10.5858/arpa.2013-0572-OA.


Context: Elevated free hemoglobin (Hb) and bilirubinemia complicate extracorporeal membrane oxygenation and could affect unfractionated heparin (UH) therapy monitoring by anti-Xa assay and activated partial thromboplastin time (aPTT).

Objectives: To compare in vitro response of anti-Xa and aPTT assays to UH in samples with artificial hyperbilirubinemia and hyperhemoglobinemia and to estimate if this interference is also observed in vivo in pediatric extracorporeal membrane oxygenation.

Design: Measurement of aPTT and anti-Xa activity in plasma spiked with UH and increased concentration of free Hb and/or conjugated bilirubin. All samples with anti-Xa activity, antithrombin, free Hb, and bilirubin determination and infused dose of UH from inpatients on extracorporeal membrane oxygenation were extracted from the clinical patient database and analyzed.

Results: Each increment of free Hb by 100 mg/dL significantly shortened aPTT, whereas an increment of bilirubin by 6 mg/dL caused significant prolongation of aPTT and stepwise increase of free Hb and/or bilirubin in plasma decreased anti-Xa activity by 0.03 to 0.05 IU/mL. Extracorporeal membrane oxygenation samples with free Hb 50 mg/dL or greater had significantly lower anti-Xa activity compared with normal ones: 0.33 (0.25-0.42) versus 0.4 (0.31-0.48) IU/mL (P = .01), despite the identical UH infusion and similar antithrombin activity. Moderate increase of free Hb by 59 mg/dL was associated with absolute decrease of anti-Xa activity by 0.07 IU/mL.

Conclusions: Activated partial thromboplastin time and anti-Xa assay are affected by elevated level of free Hb and/or bilirubin in the presence of UH, and lower anti-Xa activity is noted in extracorporeal membrane oxygenation patients with elevated free Hb. Severe hemolysis and/or hyperbilirubinemia could compromise UH monitoring based on these assays.

Publication types

  • Observational Study

MeSH terms

  • Anticoagulants / administration & dosage
  • Anticoagulants / blood
  • Bilirubin / blood*
  • Child, Preschool
  • Extracorporeal Membrane Oxygenation
  • Factor Xa Inhibitors / blood*
  • Female
  • Hemoglobins / analysis*
  • Heparin / administration & dosage
  • Heparin / blood*
  • Humans
  • Infant
  • Infant, Newborn
  • Male
  • Partial Thromboplastin Time*
  • Prospective Studies


  • Anticoagulants
  • Factor Xa Inhibitors
  • Hemoglobins
  • Heparin
  • Bilirubin