A non sequitur is not always apparent. Although obvious in the syllogism, "birds sing, Smith sings, ergo Smith is a bird," it may escape notice in the plausible, "men sing, Smith sings, therefore Smith is a man." In similar fashion, plausibility often disguises a non sequitur in the interpretation of laboratory data. The conclusion that Smith is thyrotoxic may be correct, but if based solely on the finding of a high protein bound iodine value, it is a fallacy; nonthyrotoxic subjects who have ingested iodine or have received radiopaque iodinated dyes also manifest this characteristic. In fact, every false-negative or false-positive test is a non sequitur.
Sabath et al1 call attention to the non sequitur of interpreting elevated serum glutamic oxaloacetic transaminase (SGOT) values in patients receiving erythromycin as indicative of liver damage. Having observed marked changes in SGOT levels obtained by a colorimetric method in which a diazonium