Loss of coproporphyrin by centrifuging urine samples before liquid-chromatography

We have previously reported a "high-performance" liquid-chromatographic procedure for urinary porphyrins. While attempting to automate this procedure, we discovered that coproporphyrin could be partly or completely lost from urine samples if they were centrifuged or filtered before analysis. The loss is related to the amount of precipitate in each sample, acidic urines showing the greatest losses. If samples are alkalinized before centrifugation, the loss is prevented.