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Diagnosing and identifying the type of porphyria in a patient requires specialist analysis of porphyrins and/or precursors in blood, urine and faeces. Correct sample storage and handling prior to analysis is essential to minimise preanalytical error.We evaluated the impact of light exposure, time and temperature on erythrocyte and plasma porphyrins in samples from patients with erythropoietic protoporphyria (EPP) and porphyria cutanea tarda (PCT) stored as whole blood for up to 96 hours, and in addition, the effect of freeze-thaw on plasma porphyrins in EPP, PCT and hereditary coproporphyria patient samples.Plasma porphyrins in the EPP patient samples decreased on average by 19% after 6 hours despite light protection and fridge storage, 36% by 24 hours stored light protected at room temperature, 67% within 1 hour when light exposed and 33% after one freeze-thaw cycle. In contrast, plasma porphyrin in PCT samples demonstrated greater stability compared to the EPP samples when stored light protected or exposed at room temperature and during freeze-thaw. Erythrocyte porphyrins in EPP samples were stable for 96 hours under all three storage conditions examined.Erythrocyte protoporphyrin analysis should be undertaken as an additional first-line investigation alongside plasma porphyrin analysis whenever protoporphyria needs to be excluded, due to the significant instability of plasma protoporphyrin.