Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (655 page)

   Screen of urine: indication of exposure

   Screen of serum: used to assess potential toxicity

   
Normal range:
5–20 μg/mL serum

   
Potentially toxic:
>150 μg/mL measured 4 hours postdose

   Limitations

   Screening

   Serum/urine: colorimetric or immunoassay on automated chemistry analyzers

   High bilirubin concentrations [>50 μg/mL] may cause false-positive results with immunoassay-based tests.

   Plasma may be tested in place of serum. Anticoagulants such as EDTA and heparin do not generally interfere with the assay.

   Do not use whole blood.

   Confirmation:

   Serum/urine–HPLC or GC/MC

   APAP is highly conjugated by glucuronidation and sulfation.