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Technical Briefs |
Departments of1
Clinical Biochemistry,2
Nuclear Medicine, and4
Haematology, East Kent Hospitals National Health Service Trust, Kent and Canterbury Hospital, Canterbury, Kent, UK;
3 South West Thames Institute for Renal Research, St. Helier Hospital, Carshalton, Surrey, UK;
aaddress correspondence to this author at: Department of Clinical Biochemistry, East Kent Hospitals NHS Trust, Kent and Canterbury Hospital, Canterbury, Kent CT1 3NG, UK; fax 44-01227-783077, e-mail edmund.lamb{at}ekht.nhs.uk
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 study, we recruited 39 Caucasian volunteers with a confirmed diagnosis of multiple myeloma through hematology outpatient clinics. The study was approved by the Local Research Ethics Committee. Patients were prospectively enrolled from April 2001 to February 2003 during times the investigators were available. Exclusion criteria were active rheumatoid disease, renal dialysis, or renal transplantation. Patients attended the hospital for a 51Cr-EDTA clearance within 1 month of recruitment, bringing with them a 24 h urine collection. Blood was taken for serum cystatin C, ß2-microglobulin, urea, albumin, and creatinine measurement. Patients were not asked to fast on the day of attendance. Descriptive data for the patients are given in file 1 of the Data Supplement that accompanies the online version of this Technical Brief at http://www.clinchem.org/content/vol50/issue10/.
GFR was estimated from a single 51Cr-EDTA injection and three blood samples by mono-exponential analysis with the Brochner-Mortenson correction (27). GFR measurements were undertaken by technologists/scientists and reported by accredited specialist physicians working in a nuclear medicine department accredited for training purposes by the Institute of Physical Sciences in Medicine. Serum and urinary creatinine were measured by a kinetic Jaffe method (Integra 800 analyzer, Roche Diagnostics) calibrated with the manufacturer’s recommended human-serum-based material: between-day imprecision was <3% at concentrations of 113 and 333 µmol/L. Serum cystatin C was measured by a particle-enhanced nephelometric immunoassay (Dako Ltd.) on an automated rate nephelometer (ImmageTM analyzer; Beckman-Coulter) (28): between-day imprecision was 6.6% at 0.97 mg/L and 2.6% at 5.53 mg/L. Absence of interference from paraproteins had been confirmed (29). Serum ß2-microglobulin was measured by a latex particle-enhanced turbidimetric immunoassay (Binding Site Ltd.): between-batch imprecision was <8%. Serum albumin and urea were measured by bromcresol green and urease methods, respectively (Integra 800 analyzer). Creatinine clearance, Cockcroft and Gault (9), and MDRD (10)(11) calculated clearances, body mass index [weight (kg)/height (m)2], and body surface area (30) were calculated by use of standard formulas. 51Cr-EDTA, measured and calculated clearance GFR estimates were all adjusted to a mean surface area of 1.73 m2. Paraprotein typing was confirmed by studying serum and urine samples by use of agarose gel electrophoresis and immunofixation (Hydrasys; Sebia), and paraprotein concentrations were determined by densitometric scanning. Analyses were undertaken in an accredited (CPA UK Ltd.) laboratory by state-registered biomedical scientists blinded to the 51Cr-EDTA results.
Data were analyzed by Analyze-itTM software (Analyze-it Software Ltd.). 51Cr-EDTA was the reference method: log-transformed values were used because this gave a closer approximation to a gaussian distribution for this variable. The analysis was complicated by the known nonlinear and inverse relationship between serum markers of GFR (creatinine and cystatin C) and GFR itself, compared with the other clearance estimates, which are expressed in the same units as GFR (creatinine clearance and formulaic estimates of clearance). To enable comparison, reciprocal and log-transformed analyses were also calculated. For creatinine clearance and the formulaic estimates of clearance, comparison with 51Cr-EDTA clearance was also undertaken by use of difference plots, and the bias and imprecision of the estimates were compared by the paired t-test and F-test, respectively.
Using the relevant reference interval for the kidney function tests (see file 1 in the online Data Supplement), we assessed sensitivity for the detection of moderate kidney disease against a recently proposed threshold of 60 mL · min–1 · (1.73 m2)–1 (12). Differences in the sensitivities of the test markers for renal dysfunction were tested by the Fisher exact probability test (2-tailed P values are given). The existence of a relationship between cystatin C and tumor burden was tested by comparing cystatin C concentrations against (a) serum ß2-microglobulin and (b) paraprotein concentration and (c) by comparing cystatin C concentrations in patients with stage I, II, and III disease.
The R2 of linear regression analyses comparing the test marker (y) with 51Cr-EDTA (x) were 0.91 for Cockcroft and Gault and simplified MDRD clearances, 0.90 for MDRD clearance and creatinine clearance, 0.86 for serum creatinine, and 0.73 for cystatin C (Table 1
).
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On average, creatinine clearance and the three formulaic estimates of clearance all showed a slight positive bias compared with 51Cr-EDTA (P <0.05 in all cases; Table 1
). In terms of the precision of the estimate of GFR, the 95% limits of agreement [mL · min–1 · (1.73 m2)–1 in all cases] did not differ (P >0.05) between the simplified MDRD (43.5), MDRD (47.3), Cockcroft and Gault (50.4), and measured creatinine clearance (50.9) estimates (Table 1
).
There was no relationship (P >0.05) between serum paraprotein concentration and either log-transformed GFR (R2 = 0.02) or cystatin C (R2 = 0.03). We observed a significant positive relationship between serum cystatin C and ß2-microglobulin concentrations [R2 = 0.66 (P <0.0001); ß2-microglobulin = 3.704(cystatin C) – 0.44]: however, when ß2-microglobulin concentration was adjusted for glomerular dysfunction (by expressing as a ratio to serum creatinine), this relationship was abolished (R2 = 0.00). There were no significant differences (P >0.05) between the cystatin C/creatinine ratios in patients with stage I, II, or III disease. There was a significant relationship between serum cystatin C and serum creatinine [R2 = 0.77 (P <0.0001); cystatin C = 0.0077(creatinine) + 0.56].
GFR was <60 mL · min–1 · (1.73 m2)–1 in 20 of 39 patients, but serum creatinine was increased above the reference interval (i.e., true positive for moderate kidney disease) in only 8 of 20, compared with 20 of 20 for cystatin C (P <0.0001; Fig. 1
). Cockcroft and Gault- and MDRD-calculated clearances were <60 mL · min–1 · (1.73 m2)–1 in 15 of 20 patients, both of which appeared to be inferior to cystatin C for detecting moderate kidney disease (P = 0.0471). The simplified MDRD and measured creatinine clearances detected 16 of 20 of these patients (P >0.05 compared with cystatin C).
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Among 19 patients with GFR 
60 mL · min–1 · (1.73 m2)–1, 10 had serum cystatin C concentrations exceeding the reference interval (i.e., false positive for moderate kidney disease) compared with none for serum creatinine (P = 0.0004). All 19 had MDRD and simplified MDRD clearance estimates >60 mL · min–1 · (1.73 m2)–1 (P = 0.0030): one measured creatinine clearance and one Cockcroft and Gault clearance estimate was <60 mL · min–1 · (1.73 m2)–1 (P = 0.0004). (P values are compared with cystatin C in all cases.) There were no particular distinguishing clinical features (drug history, presence of free light chains, skeletal lesions, or comorbid conditions) in the patients with increased serum cystatin C concentration and GFR 60 mL · min–1 · (1.73 m2)–1.
Finney et al. (31) also demonstrated a strong correlation between serum cystatin C and creatinine in myeloma patients (r = 0.89 compared with r = 0.88 in the present study), but an estimate of GFR was not reported by these authors. They also observed no relationship between serum cystatin C and disease burden. Tumor burden probably does not exert a strong influence on serum cystatin C concentration independently of the effect of myeloma itself on kidney function. Given this, does cystatin C measurement confer any advantages in the assessment of kidney function in these patients compared with conventional markers? Cystatin C was more sensitive than serum creatinine at detecting moderate reductions in GFR, and we observed a good correlation with GFR, although the strength of this relationship was lower than that observed with all other markers tested, including creatinine clearance. The good performance of creatinine clearance in this setting is unusual: in nearly all other studies, the performance of creatinine clearance was inferior to that of serum creatinine and calculated estimates of GFR (12)(32) and serum cystatin C (17)(21)(33). Many of our patients had been receiving medical care of their disease for some time: they may have been more familiar, and consequently competent, with the 24-h urine collection procedure. All calculated clearance estimates gave reasonable R2 coefficients against 51Cr-EDTA and showed reasonable sensitivity and specificity for the detection of moderate kidney disease.
In summary, cystatin C appears to reflect GFR in patients with multiple myeloma, as observed in other clinical settings, but calculated estimates of GFR appear to provide equally good information at much lower cost. In keeping with current best practice guidelines (12), we recommend that calculated estimates of GFR should supplement serum creatinine measurement alone and replace measured creatinine clearance.
Acknowledgments
We thank Wendy Van Der Steen of the Nuclear Medicine Department for the 51Cr-EDTA measurements. We are grateful to Drs. Y. Williams, A. Borg, R. Gale, and C. Pocock, from the Department of Hematology, for assistance with patient recruitment. We would also like to thank the staff of the Clinical Biochemistry Department, Kent and Canterbury Hospitals, for their cooperation and help. We are grateful to Dr. J. Sheldon from the Protein Reference Unit, St. George’s Hospital, London, for the ß2-microglobulin measurements. This work was funded by the South East Regional NHS Project Grant Scheme (Grant Reference SEO 150).
References
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) and 51Cr-EDTA clearance in patients with multiple myeloma.