Salicylates (Aspirin)

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Subject: Salicylates (Aspirin)

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Definition

  • An acidic drug that is rapidly metabolized to an active metabolite, salicylate; also known as acetylsalicylic acid (ASA). Included in aspirin, sodium salicylate, oil of wintergreen, methyl salicylate.

  • Normal therapeutic range (serum):

    • Analgesic/antipyretic use: <60 μg/mL

    • Anti-inflammatory use: 150–300 μg/mL

Use

  • ASA and salicylate have analgesic, antipyretic, and anti-inflammatory properties; used in the treatment of RA.

  • ASA also inhibits platelet aggregation and, therefore, prolongs bleeding time.

Interpretation

  • See discussion of salicylate poisoning in Chapter 14.

Limitations

  • Requires specific test request; not usually detected in routine screens:

    • Color test:

      • Limit of detection: 40–50 μg/mL

      • Suitable for blood, serum, plasma, urine

      • Not recommended for quantitative purposes due to interferences from metabolites, AND plasma constituents

  • Immunoassay

    • Serum/plasma.

    • With a Limit of quantitation: 50 μg/mL.

    • Not whole blood.

    • Anticoagulants (heparin, citrates, oxalates, EDTA) do not interfere.

    • Do not use sodium azide in collection tubes.

    • Hemolysis produces false-negative results.

    • Hemoglobin concentrations >100 mg/dL may interfere.

  • Confirmation

    • Sample pretreatment required

    • Gas chromatography: derivatization may be necessary

    • HPLC: preferred technique for distinguishing metabolites

    • Limit of quantitation: 50 μg/mL

 
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