Diagnostic Accuracy of Cystatin C as a Marker of Kidney Disease in Patients with Multiple Myeloma: Calculated Glomerular Filtration Rate Formulas Are Equally Useful

Renal impairment is a common complication of multiple myeloma (1)(2)(3). Standard assessment of kidney function in myeloma patients includes serum creatinine and, in those found to have significant renal impairment, creatinine clearance. This probably underestimates the prevalence of kidney disease. The availability of an improved measure of kidney function would aid in the selection of chemotherapy, improve monitoring of kidney function during bisphosphonate treatment, enable detection of kidney disease at an earlier stage, and improve avoidance of potentially nephrotoxic drugs.The limitations of serum creatinine as a marker of the glomerular filtration rate (GFR) are widely appreciated (4)(5). Creatinine clearance may be more sensitive, but it requires a timed urine collection, which is imprecise (6) and inconvenient (7). For clinical purposes, 51Cr-labeled EDTA clearance provides a surrogate gold standard measure of GFR (6)(8), but it is time-consuming, expensive, and not readily available in many hospitals.Attempts to improve clinical measurement of GFR include the use of creatinine-based formulas, including those proposed by Cockcroft and Gault (9) and Levey and coworkers [Modification of Diet in Renal Disease (MDRD) formula (10) and its simplified version (11)]. These improve GFR estimation compared with serum creatinine alone (12), although considerable limitations persist. Cystatin C is a 13-kDa protein whose plasma concentration reflects GFR. Its superiority over serum creatinine in terms of diagnostic sensitivity for reduced GFR is generally accepted (13)(14)(15)(16)(17)(18)(19)(20)(21), but concerns have been expressed that it may be affected by malignant progression (22)(23)(24)(25)(26). To our knowledge, neither cystatin C nor the MDRD formulas have been evaluated against a gold standard measure of GFR in patients with multiple myeloma.For this …