False proteinuria in patients with alkaptonuria.

False proteinuria in patients with alkaptonuria Sirs, Alkaptonuria (AKU) is a rare autosomal re-cessive disorder characterised by the inability to metabolise homogentisic acid (HGA). In the absence of HGA oxidase, HGA is rapidly oxidised to benzoquinone acetic acid, which polymerises to a melanin-like pigment and deposits in connective tissue. Mus-culoskeletal involvement (ochronosis) is the most serious complication of this condition (1-3). Affected individuals excrete HGA in the urine, causing a characteristic dark colour when the urine is alkalinised or oxidised. This metabolic impairment has not been associated with kidney diseases, except for an increased occurrence of stone formation (4). However, as kidneys play a critical role in eliminating HGA, preserving renal function is crucial in alkaptonuria patients. Conversely , altered renal function might lead to a faster progression of ochronosis (5). Herein we describe the case of a 45-year-old woman affected by alkaptonuria/ochro-nosis, with asymptomatic, unexpected " proteinuria ". The patient was referred to our Department for a routine checkup. Her past history was unremarkable except for her metabolic disorder, diagnosed when she was 35 years old. Over the previous 4 years she had complained of a non-specific back pain and lumbar stiffness in the flexion-extension movements, due to ochronotic involvement of the spine. No peripheral joint involvement was observable. Brownish spots of the sclerae were also evident. She had been taking ascorbic acid 1gr daily and analgesics occasionally. Biohumoral analyses including renal functional parameters and the urine dipstick test were within normal limits. However, 3 g/lt of protein were detected in the 24-hour urine sample using the benzethonium chloride method. On the basis of these conflicting results, a urine electrophoresis assay was performed and revealed that, in line with the dipstick test, proteins were absent. Renal ultrasonography did not show either parenchymal abnormalities or kidney stones. Similar results were recorded in another two ochronotic patients, who had 2.8 and 3.5g/lt of protein respectively in their 24-hour urine as measured by the benzetho-nium chloride method, but no protein according to the dipstick test or electrophore-sis assay. In order to check the possibility of falsely elevated total urinary protein, a urine sample from a healthy volunteer was analysed by the dipstick and benzethonium chloride methods. In basal conditions both tests resulted negative for proteinuria. However, the addition of HGA (5mg/ml) to the same sample confirmed the absence of protein using the dipstick test, while the benzethonium chloride method found 1 g/lt …