Background Most patients with porphyria cutanea tarda (PCT) may be successfully treated to remission, but are at risk of relapse. In Norway, patients are therefore recommended yearly follow-up including measurement of urinary porphyrins in order to identify a biochemical relapse prior to symptoms. However, there is little data on the risk of relapse and the usefulness of yearly controls. The aims of this study were therefore to utilize data from the Norwegian Porphyria Registry (NPR) to describe the clinical presentation, treatment and follow-up of PCT in Norway, and to investigate the frequency of biochemical and clinical relapse.

Methods All patients with PCT are invited to participate in the NPR. The registry contains both patient- and physician reported data. For this study, data on diagnosis, treatment and follow-up for all patients diagnosed with PCT in the period from July 2009–2023 were included (n = 359). To assess the relapse rate, data from patients with follow-up urinary samples drawn at least three years after the time of diagnosis were retrieved (n = 239). Patients were classified as having a biochemical relapse if the sum of uroporphyrin + heptaporphyrin ≥ 25 nmol/mmol creatinine.

Results The mean age at diagnosis was 60 years and 54% were women. Out of 352 patients where UROD sequencing had been performed, a likely disease-related variant was identified in 48%. At the time of diagnosis, 27% were smokers and 14% drank > 10 units alcohol per week. Based on patient-reported data, the following symptoms were most prevalent: blisters 92%; fragile skin 82%; red/brown urine 62%; excessive hair growth 34%; and increased skin pigmentation 28%. At the time of diagnosis, women had higher urinary porphyrin concentrations than men, with median uroporphyrin + heptaporphyrin of 621 nmol/mmol creatinine in women (n = 193) and 399 in men (n = 166), p = <.001). Phlebotomy was the most commonly treatment. In total, 239 patients had control samples analyzed more than 3 years after the PCT diagnosis was made, and the average time of study observation was 7 years. The number and frequency of control samples varied. When assessing the most recently analyzed sample, 25% (n = 60) had increased urinary porphyrin concentration, consistent with biochemical relapse, though most (n = 39) had moderately increased porphyrins (uroporphyrin+ heptaporphyrin <100 nmol/mmol creatinine).

Conclusion In this national cohort study, we have included more than 350 patients with PCT and followed them prospectively in the NPR. Increased urinary porphyrin concentrations were demonstrated in 25% of this cohort during follow-up, in samples taken more than 3 years after the diagnosis was made, when assessing their most recently submitted sample. Thus, a high proportion of the PCT patients in our study presented with biochemical relapse, indicating that routine follow-up may be beneficial to identify patients in whom closer follow-up or treatment should be initiated.