Erythrocytes of patients suffering from erythropoietic protoporphyria (EPP) contain high levels of unchelated protoporphyrin IX (PP) molecules when they enter circulation, and the leakage of PP that leaks from the circulating cells is responsible for the patients' cutaneous photosensitivity. The level of PP in EPP blood has long been used as an indicator of the severity of the disease and is useful in its management. The present study investigates what additional information may be obtained by determining the distribution of the PP content of individual EPP red cells. Absorption and fluorescence images of fields of the dispersed and immobilized red cells from nine patients with EPP were acquired under computer control by use of an inverted fluorescence microscope equipped with a cooled slow-scan charge-coupled device camera. The distribution functions of the fluorescence emitted by individual red blood cells (IRBC) were derived by a suitable image analysis program and were converted to the distributions of the cellular PP content by relating the average value of the distributions (Iav) to the PP level of packed cells, as determined by an extraction assay. The IRBC distributions show that a small percentage of the red cells is responsible for most of the PP fluorescence, and the distributions of IRBC/Iav for the nine patients with EPP were found to be very similar. This is consistent with the leakage rate during circulation being approximately proportional to the cells' PP content.
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