Nephrology for MRCPI Part 2

A 61-year-old man with newly diagnosed myeloma is admitted with acute kidney injury. Creatinine 380 micromol/L, calcium 3.2 mmol/L, and urine dipstick is negative for protein despite a urine protein:creatinine ratio that is markedly raised. What explains the discrepancy between the dipstick and the protein:creatinine ratio?

1. The dipstick detects albumin, whereas the excess protein here is monoclonal free light chains correct Correct. Urine dipsticks are designed to detect albumin and are insensitive to immunoglobulin free light chains. In myeloma the filtered protein is predominantly light chain, so the dipstick reads negative while quantitative measurement is grossly abnormal. This is why urine electrophoresis or serum free light chain assay is required.
2. Hypercalcaemia interferes with the dipstick reaction Wrong. Calcium does not interfere with the protein pad.
3. The patient has a false-positive protein:creatinine ratio caused by the acute kidney injury Wrong. Acute kidney injury does not generate spurious proteinuria on quantitative measurement.
4. The dipstick has expired or was read too late Reading a dipstick too late produces false POSITIVES, not false negatives, and it would not explain a systematic discrepancy of this kind.
5. The urine sample was too dilute to register on the dipstick Wrong. Dilution would lower the ratio as well, since creatinine falls proportionately. The ratio is specifically designed to correct for concentration.

The point: A negative dipstick with heavy quantitative proteinuria is a myeloma clue. Cast nephropathy is the commonest renal lesion; hypercalcaemia, dehydration, NSAIDs and contrast compound it. Treat with fluids, correct the calcium, stop nephrotoxics, and start myeloma-directed therapy urgently, renal recovery depends on rapid light chain reduction.

Source: BSH guidelines on the diagnosis and management of multiple myeloma British Society for Haematology · tier 2, specialty society or college