Prognostic significance of serum cystatin c concentrations in renal transplant recipients: 5-year follow-up

Prognostic Significance of Serum Cystatin C Concentrations in Renal Transplant Recipients: 5-Year Follow-Up T.D. Leach, C. Kitiyakara, C.P. Price, J.M. Stevens, and D.J. Newman

C

YSTATIN C (CyC) is a 13.6kD molecular-weight, nonglycosylated basic protein of family 2 of the cystatin superfamily of cysteine protease inhibitors.1 It is synthesized at a constant rate by all nucleated human cells. The structure of the CyC gene and its promoter suggest they are of the housekeeping type. The low molecular weight of CyC, and its stable rate of production, suggest that serum concentrations (sCyC) are determined mainly by glomerular filtration.2 A low-molecular-weight protein should be freely filtered at the glomerulus, and either excreted in the urine or metabolized by the tubules.3 If the production of the protein remains constant, the serum concentration should reflect the glomerular filtration rate. Many studies during the last few years observed a correlation between sCyC and glomerular filtration rate,4 –7 including studies in renal transplant patients.8 –12 None of these investigations established the prognostic value of sCyC measurements. The measurement of glomerular filtration rate has been investigated with other low-molecular-weight proteins, namely, ␤2-microglobulin, retinol-binding protein, and ␣1microglobulin (protein HC).1,7 However, the influence of nonrenal factors including infection, dietary factors, or liver disease on the serum concentrations of these proteins has made their use impractical. The smaller molecules, such as creatinine (Cr) and urea, more commonly used for the clinical assessment of the glomerular filtration rate, are affected by muscle mass, protein intake, gender, age, and extrarenal metabolism.13 There are also several well-known difficulties in the analytical measurement of Cr.14 Therefore, there is a need for a more reliable and more easily analyzed measurement of glomerular filtration rate. SCyC has been shown to be at least as good a measure of glomerular filtration rate (GFR) as sCr.4 –12 There has been evidence presented that sCyC is unaffected by malignancy or infection.1,15 Several investigators have devised analytical methods for quickly and reliably measuring sCyC, and latex particle– enhanced immunoturbidimetric assays have proven accurate and valid.16 –18 Studies have revealed the normal population range of sCyC, which after the age of 1 year is remarkably constant, and not significantly different between genders.18 –20 The effect of reduced glomerular filtration rate on sCyC has also been studied. In patients undergoing unilateral nephrec-

tomy, sCyC doubles after the removal of the kidney, whereas sCr may remain within the normal range.21 It is known that sCyC is not affected by low-flux hemo˚ ), age, gender, or muscle dialysis (MW 13.6 kD, Mr 15.1 A mass.19,22,23 In hemodialysis patients the only influences on sCyC are residual diuresis and the use of high-flux membranes. Clearance of CyC is low in CAPD (1.6 mL/min 䡠 1.73 m2), causing elevated serum concentrations.24 In comparison with Cr, there are physiologic differences in the renal handling of CyC.2 Because of its relatively high molecular weight and tubular metabolism, once CyC is filtered at the glomerulus it does not return to the circulation.25 Thus, as long as glomerular filtration is preserved, sCyC remains stable. By contrast, filtered Cr may leak back into the blood from the tubules especially in the presence of acute tubular injury.14 This movement increases sCr, leading to an underestimation of the GFR. Cr but not CyC is also secreted by the cationic organic acid pathway,26 a mechanism that displays a large interindividual variation, and is inversely proportionate to GFR. Cross-sectional studies in patients with stable transplant function have indicated that sCyC discriminates an abnormal GFR (as determined by clearance techniques) more sensitively than sCr in the transplant population.12,27–29 Three other studies have examined the changes in sCyC in the immediate posttransplant period (one in pediatric patients30).8,9 However, none has investigated the role of sCyC in predicting graft survival. The object of this study was to investigate further the role of serial monitoring of sCyC in the immediate posttransplant period, particularly in the complicated patient, and to determine its role in predicting long-term graft survival (compared with sCr). SUBJECTS AND METHODS All 21 patients receiving a renal transplant in the Wessex Renal & Transplant Unit between August 1 and October 31, 1994 were From the Wessex Renal & Transplant Unit St Mary’s Hospital, Portsmouth, UK (T.D.L., C.K., J.M.S.); and Department of Clinical Biochemistry, St Bartholomew’s School of Medicine and Dentistry, London, UK (C.P.P., D.J.M.). Supported by a research grant from the Renal Research Fund, Wessex Renal & Transplant Unit (charitable funds). Address reprint requests to Dr Timothy D. Leach, Wessex Renal & Transplant Unit, St Mary’s Hospital, Portsmouth, UK.

0041-1345/02/$–see front matter PII S0041-1345(02)02818-X

© 2002 by Elsevier Science Inc. 655 Avenue of the Americas, New York, NY 10010

1152

Transplantation Proceedings, 34, 1152–1158 (2002)

SERUM CYSTATIN

1153 Table 1. Preoperative Patient Details

Patient No.

Age (Years)

Gender

Primary Disease

1 2 3 4 5 6 7

24 64 69 43 66 71 52

M M M M M F M

Mesangiocapillary GN Mesangiocapillary GN Obstructive uropathy Polycystic kidney disease Hypertensive nephropathy Polycystic kidney disease Diabetic nephropathy

8 9

42 21

M M

Diabetic nephropathy Reflux nephropathy

10 11 12 13 14 15 16 17 18 19 20 21

44 48 60 35 67 25 57 67 45 66 51 49

M F F M M M M M F M F M

Reflux nephropathy Glomerulonephritis Polycystic kidney disease IgA nephropathy Diabetic nephropathy Diabetic nephropathy Diabetic nephropathy Hypertensive nephropathy Reflux nephropathy Mesangiocapillary GN Polycystic kidney disease Chronic interstitial nephritis

Mode of Dialysis

Weight (kg)

Residual Urine Output (mL/d)

Initial CyC (mg/L)

CAPD HD HD HD HD HD PreESRF HD PreESRF HD HD HD CAPD CAPD CAPD CAPD CAPD CAPD CAPD CAPD HD

65.0 75.0 88.7 71.7 69.5 58.0 63.7

Not recorded Not recorded Not recorded 0 0 500 600

7.19* 7.88 8.80 6.55 10.02 5.60 4.55

81.5 66.7

50 Not recorded

9.62 7.30

73.8 60.1 71.0 91.9 88.0 66.7 79.4 72.1 47.9 65.4 63.0 62.1

200 0 Not recorded Not recorded 1500 50 50 400 0 100 0 250

12.00 7.76* 7.11 4.96* 8.48 4.10* 7.75 7.90 6.85 11.10 7.40* 10.00*

CAPD, continuous ambulatory peritoneal dialysis; ESRF, end-stage renal failure; F, female; GN, glomerulonephritis; HD, hemodialysis; IgA, immunoglobulin A; M, male. *

studied. Among the 16 men and 5 women the mean age was 51 years (range 21 to 71 years), 2 were pre– end-stage and 19 dialysis-dependent, of whom 10 were on hemodialysis and 9 on continuous ambulatory peritoneal dialysis. Preoperative weight and residual diuresis were recorded. Venous blood was collected in serum gel tubes (Becton Dickinson) immediately prior to the operation and at the same time each weekday (8:00 to 9:00 am) until the patient’s discharge from the transplant ward. The samples were separated by centrifugation at 2000g and stored at ⫺20°C. They were analyzed in one batch at the Royal London Hospital Department of Clinical Biochemistry, using their in-house latex particle-enhanced immunoturbidimetric assay performed on a Monarch 2000 automated analyzer (Instrumentation Laboratories, Warrington, UK).16 sCr was measured by fixedinterval Jaffe´ reaction on the same automated analyzer. The paired sCyC and sCr results were compared, and were also examined in the context of each patient’s clinical situation. Spearman’s technique was used to find the correlation of sCyC and sCr, and of sCyC with the calculated creatinine clearance (cCrCl) using the Cockcroft–Gault equation.31 This formula, developed for adults, was previously validated against 99mTc-DTPA-measured GFR in renal transplant patients.32 It was expected that a proportion of the patients would have an uneventful course after transplantation, and the remainder would experience complications. In the patients with postoperative complications, the daily changes in sCyC were compared with sCr. Using Spearman’s correlation, sCr and cCrCls of the surviving patients at 5 years posttransplantation were compared with discharge sCyC and sCr and cCrCl. Discharge sCyC, sCr, and cCrCl were also compared for those patients who lost their grafts against those whose grafts survived using the Mann–Whitney U test. In an attempt to be able to predict graft survival at 1, 3, or 5 years, cut-off values of discharge sCr, sCyC, and cCrCl values were sought by

receiver-operating characteristic curve analysis (according to the method of Henderson33). The analysis established the greatest positive and negative predictive values as well as chi-square or Fisher’s analysis of true graft survival versus that predicted by the cut-off values. The statistical operations were performed using SPSS (version 10) for Windows on a PC-compatible computer.

RESULTS

Table 1 gives the details of the 21 patients who participated in the study. Preoperative daily urine volume was not recorded in six patients. Postoperative values were used as the initial sCyC for patients denoted by superscript “*” in Table 1. The initial sCyC value was not significantly different between genders, between the dialysis modalities, or between those patients not yet dialysis-dependent and those on dialysis. There was no significant correlation between patients weight and initial sCyC. In all, 306 blood samples were taken from these patients. Fig 1a shows the correlation between the reciprocal serum concentrations of sCr and sCyC, Figure 1b between 1/Cr and cCrCl, and Fig 1c between 1/CyC and cCrCl. All relationships were highly statistically significant. Nine patients had a straightforward fall in sCyC and sCr to baseline (Fig 2). Eight of the nine had no postoperative complications. The other patient (13) suffered a symptomatic bladder leak that did not interfere with graft function and was repaired uneventfully. The remaining 12 patients all experienced postoperative complications (Table 2). Examples of the sCyC and sCr

1154

LEACH, KITIYAKARA, PRICE ET AL

Fig 2. Serial sCr and sCyC in nine patients without delayed fall of sCr and sCyC posttransplant. The values fall quickly to baseline, although the levels of sCyC are proportionately greater than sCr when compared with normal. Circles with dashed lines: sCyC, triangles with solid lines: sCr.

Fig 1. Overall relationships between reciprocal sCyC and sCr and cCrCl in all subjects: (a) reciprocal sCr versus sCyC; (b) reciprocal sCr versus cCrCl; and (c) reciprocal sCyC versus cCrCl.

responses to several common posttransplantation complications are indicated in Figs 3 to 6. Table 3 shows the postoperative results, including patient and graft survival data. There was no correlation between discharge sCyC or sCr with the 5-year sCr or cCrCl. Discharge sCyC tended to be lower for grafts that survived than for grafts that did not. This difference was significant for grafts that survived 3 years and those that did not (sCyC

in survivors vs nonsurvivors: at 1 year mean rank ⫽ 9.7 vs 15.2, Z ⫽ ⫺1.73, P ⫽ .083; at 3 years, 9.3 vs 15.3, Z ⫽ ⫺2.02, P ⫽ .043; at 5 years, 9.9 vs 12.9, Z ⫽ ⫺1.09, P ⫽ .277). A similar trend was seen for differences between discharge sCr. Statistical significance was achieved between the discharge sCr for those grafts that survived 1 year and those that did not (Z ⫽ ⫺2.02, P ⫽ .043), and between those of grafts that survived 3 years and those that did not (Z ⫽ ⫺2.06, P ⫽ .039), but not at 5 years (Z ⫽ ⫺1.63). Similarly, there was a significant difference between the discharge cCrCl of surviving grafts and those that failed, achieving significance at 1 and 3 years, but not at 5 years (Z ⫽ ⫺2.19, P ⫽ .029; Z ⫽ ⫺2.22, P ⫽ .026 and Z ⫽ ⫺1.92, respectively). From a large series of patients, Teraski’s group found that discharge sCr is an important predictor of 1-year graft survival.34 In our study, the diagnostic accuracy of discharge sCyC, sCr, and cCrCl for predicting 1-year graft survival was assessed by the calculation of the areas under receiver operating characteristic (ROC) curves, a commonly used assessment.28,33 As shown in Fig 7, 1/CyC was similar to 1/Cr and 1/CrCl. To describe the characteristics of the analyzed parameters in predicting 1-, 3-, and 5-year graft survival, sensitivity, specificity, positive predictive values, and positive likelihood values were calculated (Table 4). In our patients, the 1-year graft survival of patients with a cut-off value for discharge sCyC of ⬍3.24 mg/L was 100% (n ⫽ 8), compared with 62% (n ⫽ 9/13) on or above this cut-off (Fisher’s P ⫽ NS). This difference was significant at 3 years (100% [n ⫽ 8] below vs 54% [n ⫽ 8/13] on or above, P ⫽ .032) and still apparent, although not statistically significant, at 5 years (88% [n ⫽ 7/8] vs 46% [n ⫽ 6/13]). A similar relationship was found at a cut-off value for discharge sCr of 178 ␮mol/L, with the same survival figures and statistical significance levels. In our data, the two cut-off values do not encompass the same patients, which can be seen in Table 3. A significantly smaller percentage of those

SERUM CYSTATIN

1155 Table 2. Postoperative Complications

Surgical Complications Patient Number

3 4 5 6 7 8 12 13 14 17 19 20 21 Total (n⫽21)

Wound Dehiscence

Ureteric Obstruction

⻫

⻫

Immunological Complications

Bladder Leak

Acute Cellular Rejection—BiopsyProven

Acute Cellular Rejection—Presumed and Treated

Other Complications Acute Vascular Rejection

⻫ ⻫ ⻫

⻫ ⻫ ⻫

Delayed Graft Function

⻫ ⻫ ⻫ ⻫ ⻫ ⻫

Urinary Tract Infection

Biopsy-Proven Cyclosporine Toxicity

⻫

⻫

⻫

2 (9.5%)

⻫ 2 (9.5%)

⻫

1 (4.8%)

⻫ ⻫ ⻫ 4 (19.0%)

⻫

⻫

3 (14.3%)

⻫ 3 (14.3%)

grafts with discharge cCrCl ⬍32.0 mL/min survived to 1, 3, and 5 years (1 year: 100% [n ⫽ 10] vs 55% [n ⫽ 6/11], P ⫽ .023; 3 years: 100% [n ⫽ 10] vs 45% [n ⫽ 5/11], P ⫽ .009; 5 years: 90% [n ⫽ 9/10] vs 36% [n ⫽ 4/11], P ⫽ .017). DISCUSSION

Our data confirm that sCyC displays no relationship to the age, gender, or weight of the patient. We showed that the

⻫ ⻫ ⻫ ⻫ 3 (14.3%)

1 (4.8%)

10 (47.6%)

sCyC value is independent of the dialysis modality used in the treatment of patients with end-stage renal failure. The posttransplantation sCyC and sCr correlate reasonably well with cCrCl values calculated using the Cockcroft–Gault algorithm. The relationships are not as good as some have described previously in comparison of sCyC with reference GFR procedures.16,35 This finding is probably due to the present data set including a mixture of patients, some of

Fig 3. Posttransplant sCr and sCyC, and daily urine outputs in 3 patients with acute cellular rejection (ACR) and delayed graft function. The variation in sCr with dialysis is not followed by sCyC. ATG, antithymocyte globulin; ATN, acute tubular necrosis; USS, ultrasound scan; HD, hemodialysis; MPP, methylprednisolone; AVR, acute vascular rejection; pyelo, acute pyelonephritis; inverted triangles, hemodialysis sessions; circles with dashed lines, sCyC; triangles with solid lines, sCr; bars, recorded daily urine output (peak values indicated in parentheses, in milliliters per day).

1156

Fig 4. Posttransplant complication: ureteric obstruction—serial sCyC and sCr, serial daily urine output. For symbols see Fig 3.

whom remain dialysis-dependent, where, as shown previously,24,35 sCyC is neither cleared readily by low-flux hemodialysis, nor does it leak back to the circulation in the absence of urine output as does Cr. In the patients with complicated posttransplant clinical courses, the relationship between sCyC and sCr is much weaker. sCyC is completely dependent on glomerular filtration and not on the existence of a urinary output. There is significant evidence in the literature that, in the absence of urine output or even in the presence of a low GFR, Cr can leak back into the circulation through damaged proximal tubules, thus underestimating the GFR.2 While proteins of the molecular weight of CyC are relatively freely filtered at the glomerular barrier, and as such can be used as markers of GFR, they cannot passively cross the proximal tubular barrier. CyC would be reabsorbed and metabolized into constituent amino acids by the proximal tubular resorptive pathways.25 As shown in Fig 3a– c, 4, and 6, despite a static or rising sCr with/without the absence of visible urine output, sCyC can fall if the glomeruli are filtering even

Fig 5. Posttransplant complication: cyclosporine nephrotoxicity—serial sCyC and sCr. For symbols see Fig 3.

LEACH, KITIYAKARA, PRICE ET AL

Fig 6. Posttransplant complication: ureteric dehiscence— sCyC and sCr, serial daily urine output. For symbols see Fig 3.

slightly. Similar results have been shown in recovering delayed graft function.9 This observation may alert the clinician, earlier than is currently achievable, to the possibility of reversible renal transplant insults. In the nine patients with an uncomplicated posttransplantation course, sCyC and sCr both fell rapidly to baseline. Mean sCr at discharge was 194 ␮mol/L, and for sCyC was 2.72 mg/L. While both these concentrations reflect less-than-normal GFRs, sCyC was more than twofold above the upper limit of normal whereas sCr was barely 50% above the upper limit of the population reference ranges. sCyC seem to reflect more closely the transplanted functioning renal mass, as does the cCrCl, the mean discharge value being 45.8 mL/min (normal 70 to 140 ml/min), confirming the findings of Le Bricon et al.8 Although firm conclusions are difficult to draw from the relatively small numbers of patients reported herein, some evidence is provided that sCyC may change more promptly than sCr in response to changes in GFR induced either by acute rejection and its treatment, cyclosporine toxicity, or complete ureteric obstruction. Furthermore, sCr is subjected to significant decreases following hemodialysis with low-flux membranes, whereas sCyC is unaffected. In Fig 3a, sCyC is shown to fall rapidly in the immediate posttransplant period when the patient remained dialysisdependent. sCyC reached a plateau several days prior to biopsy-proven evidence of acute cellular rejection. It then fell in response to the course of ATG. In Fig 3b, the lack of a decrease in sCyC is in accord with the presence of acute cellular rejection as diagnosed at day 7, and in Fig 3c the rise in sCyC could be followed in the intradialytic periods well before biopsy at day 8 that confirmed acute cellular rejection, with sCyC falling in response to the ATG therapy. However, in cases of complete ureteric obstruction, as shown in Fig 6, back pressure significantly reduced GFR and therefore filtration of CyC, and it was not until the clot was removed by open surgery that GFR rose and sCyC (and eventually sCr) fell. In the patient’s history summarized in

SERUM CYSTATIN

1157 Table 3. Postoperative Results

Patient Number

Discharge sCyC (mg/L)

Discharge sCr (␮mol/L)

Discharge cCrCl (mL/min)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

2.23 2.27 3.24 2.81 5.20 7.75 2.86 4.09 2.27 3.31 2.29 5.43 3.64 5.62 2.95 2.19 5.21 3.29 4.10 4.85 4.05

199 160 253 167 583 922 158 827 161 166 140 409 253 357 155 152 593 364 178 266 336

44.6 41.9 29.3 49.0 10.4 5.3 41.7 11.4 58.0 50.2 48.5 17.0 44.9 21.2 58.2 51.0 10.4 15.3 32.0 25.9 19.8

Fig 5, where biopsy evidence of cyclosporine toxicity was obtained at day 7, sCyC had been rising since day 2. This suggests that sCyC might provide an earlier indication of renal damage that is not revealed by sCr in the dialysisdependent individual. Furthermore, sCyC is not subjected to marked fluctuations during low-flux hemo- or peritoneal dialysis periods. In the patient whose posttransplant period was complicated by ureteric urinary leakage (Fig 4), sCr did not fall due the reabsorption of urinary Cr within the abdomen. Consequently, the patient remained hemodialysis-dependent. By contrast, sCyC continued to fall rapidly during the dialysis-dependent period, even in the absence of a visible

Fig 7. Nonparametric ROC plots for the diagnostic accuracy of sCyC, sCr, and cCrCl in distinguishing between graft survival and failure at 1 year. The areas under the curves 1/CyC (circles), 1/Cr (triangles), and 1/CrCl (squares) are all similar (area ⫾ 95% confidence interval: 0.763 ⫾ 0.187, 0.806 ⫾ 0.194, and 0.769 ⫾ 0.301, respectively).

Life of Graft (Days)

Five-Year sCr (␮mol/L)

Five-Year cCrCl (mL/min)

Time From Operation to Death (Days)

— — 199 — 67 76 — 40 — — — — — — — 1824 — 1803 53 840 —

114 149 — 125 — — 123 — 185 174 107 90 458 289 139

86.8 52.9

115

59.6

— — 408

17.7

— — 204 — 95 375 — 70 — — — — — — — — — — 55 — —

73.6

59.4 56.1 48.8 52.0 70.8 28.2 26.0 61.5

urinary output. Indeed, the rate of fall was not accelerated by the reimplantation of the ureter, which finally enabled sCr to fall. This continued fall in sCyC even in the absence of urine output reflects the fact that CyC was completely metabolized by the renal tubules after filtration, and its clearance, unlike that of Cr, was not dependent on excretion into the urine.14,25,36 Thus, unlike the patients with rejection, complete obstruction or cyclosporine toxicity in whom a rise in both sCr and sCyC occurred as a consequence of the decline in GFR, there was a divergent response of sCr and sCyC in ureteric urine leakage, during which the GFR was likely to be unchanged. This may help to alert the clinician earlier to the possibility of a complication than just the measurement of sCr. Although there are clinical indications for the failure of a graft to function adequately, there remains the reliance on sCr measurement in the routine monitoring of renal transplant recipients. Several limitations exist in the usage of sCr as an indication of GFR. As such, alternative markers of renal function that more reliably reflect GFR are sought in the management of renal transplant recipients. Further evidence is required to fully establish the benefits of sCyC measurement in the monitoring of these patients. Recent studies have confirmed that CyC is a good marker of GFR in this population.11,12,27,29 This study indicates that, unlike sCr, sCyC changes relative to renal function in the immediate posttransplant period. sCyC, unlike sCr, is unaffected by low-flux hemodialysis and hence could be useful in monitoring the GFR in renal transplant recipients who continue to require dialysis postoperatively. In addition, sCyC may more rapidly detect complications in the immediate posttransplant period than sCr. However, numbers of subjects in this study were small, and further studies are

1158

LEACH, KITIYAKARA, PRICE ET AL Table 4. Predictive Value of Discharge Serum Cystatin C Concentration One-Year Graft Survival

Discharge CyC (mg/L)

⬍2.23 ⬍2.27 ⬍2.29 ⬍2.81 ⬍2.86 ⬍2.95 ⬍3.24 ⬍3.29 ⬍3.31 ⬍3.64 ⬍4.05 ⬍4.09 ⬍4.10 ⬍4.85 ⬍5.20 ⬍5.43 ⬍5.62 ⬍7.75

Three-Year Graft Survival

PPV (%)

NPV (%)

␹2 P value

100 100 100 100 100 100 100 89 90 91 92 92 86 80 81 76 79 80

25 26 29 31 33 36 38 33 36 40 44 50 43 33 40 25 50 100

ns ns ns ns ns ns 0.044 ns ns ns ns 0.027 ns ns ns ns ns ns

PPV (%)

100 100 100 100 100 100 100 89 90 91 92 92 86 80 75 71 74 75

Five-Year Graft Survival

NPV (%)

␹2 P value

PPV (%)

NPV (%)

␹2 P value

30 32 35 38 40 43 46 42 45 50 56 63 57 50 40 25 50 100

ns ns ns ns ns 0.04 0.023 ns ns 0.038 0.018 0.007 0.04 ns ns ns ns ns

0 50 75 80 83 86 88 78 70 73 75 77 71 67 63 59 63 65

35 37 41 44 47 50 54 50 45 50 56 63 57 50 40 25 50 100

ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns

PPV ⫽ Positive predictive value of discharge CyC value; NPV ⫽ negative predictive value; ns ⫽ not significant; (␹2 ⫽ 1 df, true vs predicted [from cutoff discharge CyC] graft survival).

necessary to confirm the value of sCyC in predicting graft complications. It was reassuring to reproduce the prognostic value of discharge sCr for graft survival, and it was of much interest to find that the same information is available from discharge sCyC. The failure to find this relationship at 5 years of follow-up may reflect the relatively small numbers in our study, or perhaps that, at that length of time from transplantation, other factors are having a greater influence on graft survival. CyC promises to be a useful marker of glomerular filtration rate following renal transplantation, particularly in patients with complications. It can at least incorporate all the advantages of sCr measurement, including reproducing the prognostic information available from discharge sCr. Further studies are required to clarify its role. ACKNOWLEDGMENT The authors thank Dr Thakkar and Mrs H. Finney for their analytical assistance, and Dade Behring for their financial support of Mrs Finney.

REFERENCES 1. Grubb A: Clin Nephrol 38(suppl 1):S20, 1992 2. Tenstad O, Roald AB, Grubb A, et al: Scand J Clin Lab Invest 56:409, 1996 3. Guyton AC: , 7th ed. Philadelphia: Saunders; p 404 4. Meier P, Froidevaux C, Dayer E, et al: Lancet 357:634, 2001 5. Fliser D, Ritz E: Am J Kidney Dis 37:79, 2001 6. Dworkin LD: Curr Opin Nephrol Hypertens 10:551, 2001 7. Donadio C, Lucchesi A, Ardini M, et al: J Pharm Biomed Anal 24:835, 2001 8. Le Bricon T, Thervet E, Benlakehal M, et al: Clin Chem 45:2243, 1999 9. Thervet E, Le Bricon T, Hugot V, et al: Transplant Proc 32:2779, 2000

10. Le Bricon T, Thervet E, Froissart M, et al: Clin Chem 46:1206, 2000 11. Risch L, Blumberg A, Huber AR: Renal Fail 23:439, 2001 12. Paskalev E, Lambreva L, Simeonov P, et al: Clin Chim Acta 310:53, 2001 13. Mitch WE, Collier VU, Walser M: Clin Sci (Colch) 58:327, 1980 14. Perrone RD, Madias NE, Levey AS: Clin Chem 38:1933, 1992 15. Finney H, Williams AH, Price CP: Clin Chim Acta 309:5–7, 2001 16. Newman DJ, Thakkar H, Edwards RG, et al: Kidney Int 47:312, 1995 17. Finney H, Newman DJ, Gruber W, et al: Clin Chem 43:1016, 1997 18. Uhlmann EJ, Hock KG, Issitt C, et al: Clin Chem 47:2031, 2001 19. Finney H, Newman DJ, Price CP: Ann Clin Biochem 37:49, 2000 20. Finney H, Newman DJ, Thakkar H, et al: Arch Dis Child 82:71, 2000 21. Newman DJ, Thakkar H, Karin OMA, et al: J Am Soc Nephrol 6:887, 1995 22. Kabanda A, Jadoul M, Pochet JM, et al: Kidney Int 45:1689, 1994 23. Norlund L, Fex G, Lanke J, et al: Scand J Clin Lab Invest 57:463, 1997 24. Kabanda A, Goffin E, Bernard A, et al: Kidney Int 48:1946, 1995 25. Jacobsson B, Lignelid H, Bergerheim US: Histopathology 26:559, 1995 26. Levey AS, Berg RL, Gassman JJ, et al: Kidney Int 27(suppl 1):S73, 1989 27. Plebani M, Dall’Amico R, Mussap M, et al: Renal Fail 20:303, 1998 28. Risch L, Blumberg A, Huber A: Nephrol Dial Transplant 14:1991, 1999 29. Herget-Rosenthal S, Trabold S, Huesing J, et al: Transpl Int 13:285, 2000 30. Bo ¨kenkamp A, Ozden N, Dieterich C, et al: Clin Nephrol 52:371, 1999 31. Cockcroft DW, Gault MH: Nephron 16:31, 1976 32. Nankivell BJ, Grunewald SM, Allen RDM, et al: Transplantation 59:1683, 1995 33. Henderson AR: Ann Clin Biochem 30:521, 1993 34. Cecka JM, Cho YW, Terasaki PI: Transplantation 53:59, 1992 35. Tian S, Kusano E, Ohara T, et al: Clin Nephrol 48:104, 1997 36. Thielemans N, Lauwerys R, Bernard A: Nephron 66:453, 1994