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Additional key phrases: drug interference; j7uorimetric detection; glucoronic acid conjugate of dipyridamoleSeveral methods have been described for the analysis of porphyrins in blood, urine and faeces, of which reversed-phase HPLC with fluorescence detection is probably the most versatile.' Every method for metabolic investigations may be subject to interference by drugs. In most instances the amounts of administered and excreted xenobiotics are much larger than the amounts of physiological compounds. This often causes huge peaks in the chromatograms, which can hardly be confused with those of normal metabolites. However, if the detection response of the xenobiotics is much less than the compounds of interest, a possible pitfall is created. Here, we describe the interference of dipyridamole having such a fluorometric response, in two different chromatographic methods for the analysis of porphyrins, leading to erroneous results. coproporphyrin 111 and two smaller peaks at the positions of coproporphyrin I and protoporphyrin IX, suggesting a coproporphyrinuria. However, protoporphyrin IX is sparingly soluble in water and is not expected in urine. Analyses of the patient's blood and faeces disclosed in faeces and in plasma (but not in erythrocytes), single large peaks with the same retention time as protoporphyrin IX.As the findings were not consistent with any known type of porphyria, another method of analysis was applied. This was HPLC method 11 which makes use of a Brownlee RPIS column

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