A causal link between the nephrotic syndrome and acute leukemia is extremely rare [1]. Most reviews have focused on either glomerular or tubular injury that has resulted in urinary protein leakage. Lysozymuria itself as the cause of a gross proteinuria has not been previously reported.

A 57-year-old man presented with a 2-week history of fever and bilateral ankle edema. His leukocyte count was 65 x 10⁹/L (70% monoblasts), and examination of a bone marrow biopsy specimen confirmed the diagnosis of acute monoblastic leukemia. His urine had tested strongly positive for protein with a routine bedside strip test (Ames Co., Miles Laboratories, Australia Pty. Ltd.). Twenty-four hour urine sampling showed 5 g of protein, assayed with an end-point colorimetric method that used pyrogallol red molybdenum and read spectrophotometrically at 620 nm. His renal function and urine microscopy results were normal.

The differential diagnosis of the patient's marked proteinuria included renal vein thrombosis and immune complex glomerulonephritis, occasionally associated with lymphoma or other malignancies [2, 3]. We did not pursue further investigations after realizing that the patient had significant lysozymemia of 159 000 U/mL (normal range, 150 to 500 U/mL) and profound lysozymuria of 101 800 U/mL in a 24-hour urine volume of 2.2 L, corresponding to a daily excretion of 223.96 MU (normal range, 65 000 to 180 000 U/d). Lysozyme activity in serum and urine was determined by a photometric method modified from that described by Litwack [4], in which 1 unit of lysozyme activity was defined as a change in absorbance of 0.001/min at 450 nm in an assay volume of 2.6 mL at room temperature; 1 µg of reference lysozyme is equivalent to 50 units. Hence, the 24-hour urinary excretion of lysozyme corresponded to 4.5 g, close to the patient's total protein excretion of 5 g. Lysozymuria and proteinuria resolved after chemotherapy was initiated.

Lysozymemia and lysozymuria are well-documented features of acute leukemia of the monocytic lineage. At very high concentrations, lysozyme causes renal tubular damage, resulting in proteinuria and hypokalemia [5]; however, renal tubular protein loss rarely exceeds 1 g/d to yield a positive dipstick reaction.

The knowledge that profound lysozymuria can mimic the nephrotic syndrome will obviate unnecessary, and sometimes invasive, investigations. In situations in which concomitant glomerular pathology is suspected, urinary albumin should be directly measured.