Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1093 page)

   Most synthetic opioids require individual specific tests for confirmation and quantitation; for potent low-dose synthetic opioids such as buprenorphine and fentanyl, the limit of quantitation is ≤1 ng/mL.

   Sample preparation required: liquid–liquid or solid-phase extraction.

   Testing methodologies: gas chromatography, HPLC, GC/MS, LC/MSn (multiple Sn).

OSMOLAL GAP

   Definition

   The osmolal gap is a mathematical concept similar to the AG that is used to detect concentration changes in osmotically active solutes rather than ion changes. The osmolal gap is calculated by subtracting the calculated osmolality from the measured osmolality.

   
Normal range:
<10 mOsm/kg.

   Use

   Osmolal gap has been used to estimate the blood alcohol. Serum osmolality increases 22 mOsm/kg for every 100 mg/dL of ethanol; therefore, estimated blood alcohol (mg/dL) = osmolal gap × 100 ÷ 22.

   Interpretation

Increased In

   Decreased serum water content:

   Hyperlipidemia (serum will appear lipemic)

   Hyperproteinemia (total protein >10 g/dL)

   Additional low molecular weight substances in the serum (measured osmolality is >300 mOsm/kg water).