title: Paraprotein interference with turbidimetric gentamicin assay.
abstract: BACKGROUND
  Gentamicin due to its low level of resistance and rapid bactericidal activity is commonly used to treat gram-negative bacteria. However, due to its toxic effects it needs to be monitored. To date, no interference has been reported with gentamicin assays.
  
  
  METHODS
  A patient with leg cellulitis and sepsis received a single dose of gentamicin and a sample was sent for gentamicin analysis. The sample showed high blank absorbance readings on Beckman DxC800 and DC800 analysers with various dilutions. A second sample was received and analysed on a Roche Cobas system to obtain a result. A third sample was received 107 hours later with the same results and this sample was then analysed neat and post ethanol precipitation on all the turbidimetric assays available on the DxC800 analyser.
  
  
  RESULTS
  The high blank absorbance was observed upon addition of the reactive reagents due to protein precipitation. Although not obvious from the patient protein results, it was shown the presence of high IgM paraprotein, 18.9 g/L (reference range 0.4-2.3 g/L) was the cause of precipitation, giving high blank readings. Of all the other turbidimetric assays, only vancomicin and valproate showed similar high blank absorbance readings. To be able to provide more rapid results it was shown ethanol could be used as a precipitant of proteins in both calibrators and patient samples with acceptable recovery.
  
  
  CONCLUSIONS
  IgM paraprotein was identified as the cause of interference with the gentamicin, vancomicin and valproate assays. Protein interference in these assays can be overcome by precipitation with ethanol.
authors: Department of Chemical Pathology, Pathology Queensland, Princess Alexandra Hospital, Brisbane, Australia.
authors: Goce
authors: G
authors: Dimeski
authors: Department of Chemical Pathology, Pathology Queensland, Princess Alexandra Hospital, Brisbane, Australia.
authors: Kendra
authors: K
authors: Bassett
authors: Department of Chemical Pathology, Pathology Queensland, Princess Alexandra Hospital, Brisbane, Australia.
authors: Nigel
authors: N
authors: Brown
mesh_terms: D000369:Aged, 80 and over; D002481:Cellulitis / Q000097:blood / Q000150:complications / Q000188:drug therapy; D011232:Chemical Precipitation; D000431:Ethanol / Q000737:chemistry; D005260:Female; D005839:Gentamicins / Q000008:administration & dosage* / Q000032:analysis* / Q000493:pharmacokinetics; D006801:Humans; D007075:Immunoglobulin M / Q000737:chemistry; D009391:Nephelometry and Turbidimetry / Q000379:methods; D010266:Paraproteins / Q000737:chemistry*; D018805:Sepsis / Q000097:blood / Q000150:complications / Q000188:drug therapy
chemical_list: D005839:Gentamicins; D007075:Immunoglobulin M; D010266:Paraproteins; D000431:Ethanol
keywords: IgM; interference; paraprotein; turbidimetry