Salicylates (Aspirin)

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 Ž ¼g/mL

Anti-inflammatory use: 150 " “300 Ž ¼g/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 Ž ¼g/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 Ž ¼g/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