BACKGROUND: Activated partial thromboplastin time (aPTT) and antifactor Xa (anti-Xa) monitoring methods for unfractionated heparin (UFH) often disagree. The extent of discordance for those with elevated bilirubin remains unclear. Our objective was to evaluate concordance between activated aPTT and anti-Xa methods for hyperbilirubinemic patients on UFH. METHODS: This was a retrospective cohort study of 26 patients hospitalized at Rhode Island Hospital between August 2014 and September 2014. Patients had at least one bilirubin measurement >5 mg/dL. After categorizing lab values, percent agreement and kappa were used to examine concordance between aPTT and anti-Xa. RESULTS: Overall percent agreement between aPTT and anti-Xa was 50%. A nontherapeutic aPTT and therapeutic anti-Xa accounted for 98% of all disagreement. Specifically, 76.7% of disagreement was due to a subtherapeutic aPTT and a therapeutic anti-Xa. Unweighted kappa was 0.141 (95%CI: 0.048-0.235). CONCLUSION: Concordance between aPTT and anti-Xa values was poor in hyperbilirubinemic patients. [Full article available at http://rimed.org/rimedicaljournal-2016-03.asp, free with no login].