An accurate assessment of proteinuria is essential for monitoring disease progression and therapeutic response in multiple myeloma (MM). Although 24-h urine collection is the gold standard for protein quantification, its practical use is often limited. The urinary protein-to-creatinine ratio (UP/C) from spot urine samples has been proposed as a convenient alternative to 24-hr UP.

This study retrospectively analyzed 53 MM patients, comparing UP/C with 24-hr urinary protein excretion (24-hr UP) and performing subgroup analyses based on renal function, serum beta-2 microglobulin (β2-m) levels, MM type and sex.

The results showed a strong overall correlation between UP/C and 24-hr UP, but UP/C tended to overestimate proteinuria. The discrepancy was minimal in patients with normal renal function but increased in females and those with renal impairment, particularly in those with elevated serum β2-m levels (≥5.5 mg/L) and Bence–Jones protein-type MM. In these cases, UP/C overestimated proteinuria more significantly.

UP/C is a useful surrogate for 24-hr UP in MM patients with normal renal function. However, in females and patients with impaired renal function, particularly those with Bence–Jones protein-type MM or elevated β2-m levels, UP/C tends to overestimate proteinuria. Therefore, clinicians should be cautious when using UP/C for assessing disease status and therapeutic response in such cases.