title: Falsely increased chloride and missed anion gap elevation during treatment with sodium thiosulfate.
abstract: BACKGROUND
  Sodium thiosulfate (STS) is used to treat calciphylaxis and cyanide poisoning, but can lead to a serious anion-gap acidosis. We suspected that the calculated anion gap in a patient treated with STS for calciphylaxis was decreased to normal by a falsely increased chloride, and we hypothesized that STS directly interfered with chloride measurements.
  
  
  METHODS
  Plasma pools were prepared with 12 concentrations of STS from 0 to 20 mmol/l. Chloride was measured in each sample on 9 analyzers: Architect 16200, StatProfile pHOx Plus, RapidLab 1265®, Vitros 350®, Advia 1800, Roche Modular, iSTAT1, RAPIDpoint 500, and Radiometer ABL735.
  
  
  RESULTS
  Statistically significant, dose-dependent increases in reported chloride concentrations were seen with all analyzers except the RAPIDpoint 500 and Vitros. The increases ranged from 5 to 75 mmol/l at the peak thiosulfate concentrations (33 mmol/l) expected in treated patients. The CLIA-allowable error of 5% was exceeded by 4 analyzers (Architect 16200, iSTAT1, StatProfile pHOx Plus, and Radiometer ABL735). The RAPIDpoint 500 showed a 3-mmol/l decrease in measured chloride over the tested range. The Vitros analyzer showed no interference.
  
  
  CONCLUSIONS
  Interference of STS in chloride measurement in several common analyzers may lead to erroneous anion-gap calculations and confound the diagnosis of STS-induced anion-gap acidosis.
authors: Department of Pathology, University of Virginia School of Medicine and Health System, Charlottesville, VA, United States. Electronic address: sw2de@virginia.edu.
authors: Scott M
authors: SM
authors: Wendroth
authors: Department of Pathology, University of Virginia School of Medicine and Health System, Charlottesville, VA, United States.
authors: Tiffany N
authors: TN
authors: Heady
authors: Department of Pathology, University of Virginia School of Medicine and Health System, Charlottesville, VA, United States.
authors: Doris M
authors: DM
authors: Haverstick
authors: Department of Pathology, Virginia Commonwealth University, Richmond, VA, United States.
authors: Lorin M
authors: LM
authors: Bachmann
authors: Department of Pathology, Washington University School of Medicine, St. Louis, MO, United States.
authors: Mitchell G
authors: MG
authors: Scott
authors: Department of Pathology, University of Virginia School of Medicine and Health System, Charlottesville, VA, United States.
authors: James C
authors: JC
authors: Boyd
authors: Department of Pathology, University of Virginia School of Medicine and Health System, Charlottesville, VA, United States.
authors: David E
authors: DE
authors: Bruns
mesh_terms: D000136:Acid-Base Equilibrium / Q000502:physiology*; D000328:Adult; D002115:Calciphylaxis / Q000097:blood / Q000188:drug therapy; D002712:Chlorides / Q000097:blood*; D004566:Electrodes; D005189:False Positive Reactions; D005260:Female; D006801:Humans; D008180:Lupus Erythematosus, Systemic / Q000097:blood; D015203:Reproducibility of Results; D012834:Silver / Q000737:chemistry; D012883:Skin Ulcer / Q000150:complications; D013885:Thiosulfates / Q000009:adverse effects* / Q000737:chemistry / Q000627:therapeutic use
chemical_list: D002712:Chlorides; D013885:Thiosulfates; D012834:Silver; C017717:sodium thiosulfate
keywords: Acidosis; Anion gap; Calciphylaxis; Chloride; Interference; Thiosulfate