Patterns of plasma concentrations and urinary excretion of salicylate in rheumatoid arthritis.

Intersubject differences in the volume of distribution, whole body clearance, and steady-state plasma concentrations of salicylic acid (SA) were studied in a series of patients with rheumatoid arthritis and healthy control subjects. The measurement of the plasma concentration of SA 12 hr after an oral dose of 1.2 gm aspirin appears predictive of the success of long-term dosage of aspirin. Concentrations below 5 microgram/ml in this single-dose test were associated with failure to achieve therapeutic plasma concentrations of SA (above 150 microgram/ml during long-term therapy with approximately 4.8 gm aspirin per day. Conversely, plasma concentrations above 10 microgram/ml in the single-dose test were associated with levels above 150 microgram/ml during long-term therapy. The volume of distribution of SA correlated poorly with body weight (r = 0.51, p less than 0.01) and did not correlate significantly with plasma albumin levels. Corticosteroids appear to induce the metabolism of SA and most subjects dosed with oral corticosteroids and aspirin 4.8 gm/day did not attain plasma levels of SA above 150 microgram/ml. The clearance of SA was greater in male than in female patients. The difference appears to be of clinical significance since fewer men than women achieved therapeutic plasma concentrations of SA.