Blue-green discoloration of urine and false nephrotic range proteinuria at dipstick urinalysis

A 32 year-old woman presented to the emergency department with symptoms of urinary tract infection. Over the past 2 days, she reported the use of a medication whose active compounds were methenamine (120 mg) + methylthioninium chloride (20 mg). A collected urine sample had a strong blue-green discoloration. Uroculture was negative and dipstick urinalysis revealed the following results: SG 1.015, pH  7, Albumin 3+, Bilirubin 2+ and Haemoglobin 2+. Urine microscopic analysis revealed 5-6 squamous epithelial cells/high power field (HPF), 5-6 leukocytes/HPF and 7-8 erythrocytes/HPF. No bacteria, cellular casts, or renal tubular epithelial cells were present in the urine sample. The most remarkable feature of the urine sediment was that some cells (squamous epithelial cells, macrophages, leukocytes and erythrocytes) were strongly stained in blue. The albuminuria measured by dipstick shows 3+ (300 mg/dL), but in turbidimetric method the urine protein concentration was 18 mg/dL, showing an important interference of methylene blue on the dipstick albumin area.