A Strong Correlation Between Spot Urine Protein To Creatinine Ratio and 24-Hour Urine Protein Quantification For Proteinuria Assessment In Patients With Plasma Cell Dyscrasia

Twenty four-hour urine collection remains one of the most crucial tools for the diagnosis and monitoring of proteinuria and quantification of urinary monoclonal protein in patients with plasma cell dyscrasias (PCD) even though it is cumbersome. Nephrology guidelines recommend replacement of 24-hour urine collection with a spot urine protein/creatinine (Pr/Cr) ratio for the measurement of proteinuria. However, only limited data exist regarding accuracy of spot urine Pr/Cr ratio in patients with PCD and utility of this measure to estimate urinary paraprotein remains uncertain. We conducted a prospective study evaluating the role of spot urine Pr/Cr ratio in patients with PCD.

From 04/2012 to 05/2013, a total of 120 PCD patients were prospectively enrolled at Moffitt Cancer Center. Oliguric patients or those requiring dialysis were ineligible. Spot urine was collected on the same day as 24-hour urine collection. Spot urine Pr/Cr ratios were compared to 24-hour urine collections with regard to the amount of (1) total protein and (2) monoclonal protein (M-spike).

Eighty four out of 120 patients (70%) were evaluable (17 did not provide spot urine samples; no Pr/Cr ratios available in 11; protein below the level of detection in 7; and one without 24-hour urine collection). Evaluable patients had a median age of 68 (range, 36 - 90) years, 63% were male, and 85% were Caucasian. Primary diagnoses were myeloma (n=74; 88%), amyloidosis (n=4), multiple plasmacytomas (n=2), solitary plasmactyoma (n=1) and MGUS (n=3). Prior therapies included chemotherapy in 95% and autologous transplant in 45%. Comorbidities included hypertension (48%), chronic kidney disease (30%), diabetes (15%), coronary artery disease (8%), atrial fibrillation (7%) and congestive heart failure (2%). The median serum creatinine was 0.9 mg/dL (range, 0.5 - 5.1). The median spot urine Pr/Cr ratio was 137 mg/g creatinine (range, 26 - 21,447). The median 24-hour urine protein was 120 mg/24h (range, 27 - 15,092), and the median urine M-spike was 1.2 mg (range, 0 - 8,099). More than half of spot urine samples were collected in the morning (59%). There were strong correlations between (1) spot urine Pr/Cr ratio and 24-hour total urine protein (Spearman correlation coefficient=0.91, p < 0.0001), and (2) spot urine Pr/Cr ratio and 24-hour urine M-spike (Spearman correlation coefficient=0.91, p < 0.0001). The timing of spot urine sample collection had no significant effects on the correlations (p = 0.46 and 0.95 by Wilcoxon rank-sum test).

Spot urine Pr/Cr ratio strongly correlates with degrees of proteinuria measured in 24-hour urine collection and may also predict the quantity of urine M-spike in non-oliguric PCD population. Spot urine Pr/Cr ratio is a potentially useful marker as a screening tool or an alternative semi-quantitative measure for rapid estimation of proteinuria which may be used for longitudinal patient follow-up in lieu of cumbersome repeated 24-hour collections.

No relevant conflicts of interest to declare.