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Osmolality gaps: diagnostic accuracy and long-term variability.

John Krahn1, Annu Khajuria

  • 1(Department of Clinical Biochemistry, St. Boniface General Hospital, and University of Manitoba Medical School, Winnipeg, Manitoba, Canada. jkrahn@sbgh.mb.ca

Clinical Chemistry
|February 4, 2006
PubMed
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The osmolal gap (OG) accurately detects toxic volatiles when controlled, but long-term stability is poor due to shifting mean OGs. Bedside calculations are not recommended.

Area of Science:

  • Clinical Chemistry
  • Toxicology
  • Laboratory Medicine

Background:

  • The osmolal gap (OG) is a critical screening tool for identifying toxic alcohol ingestions, such as methanol and ethylene glycol.
  • Assessing the diagnostic accuracy and long-term stability of OG measurements is crucial for reliable clinical application.

Purpose of the Study:

  • To evaluate the diagnostic accuracy of the osmolal gap in predicting toxic volatile levels.
  • To assess the long-term stability of osmolal gap measurements over time.

Main Methods:

  • Prospective study in 2003 and retrospective analysis from 1996-2004.
  • Calculation of OG for all volatile requests and quality control samples.
  • Receiver Operating Characteristic (ROC) curve analysis to determine predictive accuracy.

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Main Results:

  • Prospective ROC analysis showed accurate prediction of toxic volatiles, but with variable cutpoints (+10 to +33 mosmol/kg).
  • Retrospective analysis revealed a significant increase in mean OGs (12 mosmol/kg) from 1996 to 2004 across three formulas.
  • Combined data analysis resulted in poor diagnostic accuracy due to temporal shifts in mean OGs.

Conclusions:

  • The osmolal gap demonstrates high sensitivity and specificity for detecting certain volatile poisonings under controlled conditions.
  • Long-term use of the standard reference interval (-10 to +10 mosmol/kg) leads to diminished diagnostic accuracy because mean OGs are not constant.
  • Osmole gap bedside calculations are not advised due to potential inaccuracies over time.