Correlation plasma levels, NSAID and therapeutic response.

A regular control of nonsteroidal anti-inflammatory drug (NSAID) plasma levels may be useful to avoid undesirable side-effects to monitor therapeutic progress to see if patients are complying with their prescription. Trying to establish a relationship between the plasma concentration of a drug and its clinical effects requires a few prerequisites which may or may not be fulfilled according to the NSAID (e.g. a drug acting by itself, a reversible action, no tolerance to the drug, a highly specific and sensitive enough analytical method of the drug, similar free drug concentrations in the plasma and at the receptor sites,...), the most important of them-which is also probably the most difficult to fulfil in the case of rheumatic diseases-being that the clinical effect of the drug must be easily measured. In fact, the evidence for a good correlation between clinical effects and drug plasma levels are very scarce in the field of NSAID. The best correlation was obtained with salicylates for which ranges of plasma concentrations needed for observing therapeutic effects in rheumatoid arthritis as well as in juvenile rheumatoid arthritis have been established. Similar correlations have been made for side effects such as tinnitus or headaches as well as for toxic manifestations of salicylism. However, many individual variations have been described and there is considerable overlap between therapeutic and toxic concentrations. According to different authors there are or there are no correlations between phenylbutazone plasma levels and either is therapeutic or its side-effects.(ABSTRACT TRUNCATED AT 250 WORDS)