Falsely increased chloride and missed anion gap elevation during treatment with sodium thiosulfate

Scott M. Wendroth, Tiffany N. Heady, Doris M. Haverstick, Lorin M. Bachmann, Mitchell G. Scott, James C. Boyd, David E. Bruns

Research output: Contribution to journalArticlepeer-review

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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.

Original languageEnglish
Pages (from-to)77-79
Number of pages3
JournalClinica Chimica Acta
StatePublished - Apr 20 2014


  • Acidosis
  • Anion gap
  • Calciphylaxis
  • Chloride
  • Interference
  • Thiosulfate


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