title: Bi-variate approach to negative interference of bilirubin towards an acetaminophen assay.
abstract: BACKGROUND
  It is well known that enzymatic assays for acetaminophen are positively interfered by bilirubin. The effect on acetaminophen not only depends on the concentration of bilirubin but also on that of acetaminophen. We demonstrated a negative interference instead of a positive one in a commonly used routine analyzer and investigated the recovery of acetaminophen in an enzymatic assay by a bi-variate regression.
  
  
  METHODS
  Commercially available blank serum specimens were spiked with acetaminophen and bilirubin at various concentrations, and were analyzed in the Beckman Coulter AU5822 analyzer. The specimens were run in duplicates. The results were then analyzed by least-square analysis and was built into a bi-variate quadratic model.
  
  
  RESULTS
  The recovery of acetaminophen in this experiment ranged from 38.9% to 100% throughout a range of 23 μmol/L to 2052 μmol/L (for acetaminophen) and 19 μmol/L to 570 μmol/L (for bilirubin). A contour map, as well as a bi-variate equation was established, describing the relationship between acetaminophen recovery, acetaminophen concentration, and bilirubin concentration.
  
  
  CONCLUSIONS
  It was shown that the degree of bilirubin interference in a commercially available acetaminophen assay is dependent on both bilirubin and acetaminophen concentrations. There was a decrease in the apparent acetaminophen concentration by an average of 30% at a bilirubin concentration of 420 μmol/L in the Beckman Coulter AU5822 analyzer. The complex relationship can be modeled by mathematical means. This allows the laboratory staff to estimate the recovery of acetaminophen when bilirubin level is concurrently measured.
authors.affiliation: Chemical Pathology Laboratory, Department of Pathology, Princess Margaret Hospital, Hong Kong, SAR, China.
authors.forename: Yeow Kuan
authors.initials: YK
authors.lastname: Chong
authors.affiliation: Chemical Pathology Laboratory, Department of Pathology, Princess Margaret Hospital, Hong Kong, SAR, China. Electronic address: makm@ha.org.hk.
authors.forename: Chloe Miu
authors.initials: CM
authors.lastname: Mak
authors.affiliation: Chemical Pathology Laboratory, Department of Pathology, Princess Margaret Hospital, Hong Kong, SAR, China.
authors.forename: Hiu Laam
authors.initials: HL
authors.lastname: Lam
authors.affiliation: Chemical Pathology Laboratory, Department of Pathology, Princess Margaret Hospital, Hong Kong, SAR, China.
authors.forename: Man Han
authors.initials: MH
authors.lastname: Lau
authors.affiliation: Chemical Pathology Laboratory, Department of Pathology, Princess Margaret Hospital, Hong Kong, SAR, China.
authors.forename: Daniel Cheuk Wa
authors.initials: DC
authors.lastname: Leung
mesh_terms: D000082:Acetaminophen / Q000378:metabolism*; D001663:Bilirubin / Q000097:blood*; D001681:Biological Assay / Q000379:methods*; D006801:Humans; D012044:Regression Analysis; D015203:Reproducibility of Results
chemical_list: D000082:Acetaminophen; D001663:Bilirubin
keywords: Acetaminophen; Bi-variate regression; Bilirubin; Interference