Abstract Objectives The quantification of total urinary porphyrins based on spectrophotometry or spectrofluorimetry is a key screening test in cutaneous or mixed porphyria, performed before the quantitative fractionation of porphyrin isomers by chromatography. The aim of the present study was to determine the best threshold value for a spectrophotometric screening test and to estimate its diagnostic performances. Design and methods Data from samples sent to the laboratory between January 2006 and July 2013 from patients with a suspicion of cutaneous or mixed porphyria were retrospectively collected. The final diagnosis was based on the clinical presentation and biochemical results performed on appropriate specimens. Control samples were obtained from 91 non-porphyria patients and 58 healthy individuals and patient samples were obtained from 38 patients with symptomatic porphyria. The sensitivity and specificity of the spectrophotometric screening test were calculated at different cutoff values expressed in nmol/L and nmol/mmol creatinine using receiver operator curves. Results and conclusion A threshold of 18.8nmol/mmol creatinine was considered as the best cutoff value for the screening test, achieving a sensitivity of 100% and a specificity of 93.2%.