RESUMO
BACKGROUND: Purple urine bag syndrome (PUBS) can occur in cases of bacteriuria with species expressing enzymes capable of converting tryptophan metabolites to red and blue pigments which are excreted in urine, leaving a characteristic purple colour. Risk factors include urinary catheterisation, constipation and chronic kidney disease. Treatment includes catheter replacement, and antibiotics in case of urinary tract infection. CASE PRESENTATION: A man in his 70s with myelodysplastic syndrome, stage 5 chronic kidney disease and chronic indwelling urinary catheterisation due to benign prostatic hyperplasia was admitted for transfusion for symptomatic anaemia. On the second day of hospitalisation, his urine turned purple. There was no sign of transfusion reaction, haemoglobinuria, myoglobinuria or bilirubinuria. Urine cultures were positive for Proteus vulgaris and Enterococcus faecalis, two species associated with PUBS. INTERPRETATION: The constellation was consistent with PUBS. His bacteriuria was considered colonisation not requiring antibiotic treatment. The catheter was replaced and the urine colour returned to normal.