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Serum cystatin C as a marker of the renal function in patients with spinal cord injury

Spinal Cord volume 40, pages 524528 (2002) | Download Citation

Subjects

Abstract

Objective: To investigate the relationship between serum cystatin C, serum creatinine, and 51Cr-EDTA-clearance in patients with spinal cord injury.

Setting: The Spinal Cord Unit, Viborg-Kjellerup County Hospital.

Methods: Twenty-four men and seven women aged 20.3 to 68.0 years with motor complete spinal cord injury (ASIA A or B) were included. Serum cystatin C was measured by an automated particle-enhanced nephelometric immunoassay (Dade Behring), serum creatinine by an enzymatic method (Vitros 950), and 51Cr-EDTA-clearance by a multiple plasma sample method.

Results: A linear relationship was found between 51Cr-EDTA-clearance and the reciprocal values of cystatin C and creatinine. The correlation coefficient between 51Cr-EDTA-clearance and 1/cystatin C was 0.72 compared to the correlation coefficient between 51Cr-EDTA-clearance and 1/creatinine being 0.26. Comparison of the area under the curves in the non-parametric receiver operating characteristics (ROC) plots for serum cystatin C (area under the curve (AUC)=0.912; SE=0.065), and serum creatinine (AUC=0.507; SE=0.115) revealed significant differences (P-values=0.0005).

Conclusion: In patients with spinal cord injury serum cystatin C is a better marker of the renal function compared to serum creatinine.

Introduction

The measurement of serum creatinine and creatinine clearance is used as indirect markers of glomerular filtration rate (GFR). However, the concentration of serum creatinine is not an ideal parameter of GFR and glomerular filtration is only one of the parameters determining its serum concentration.1 Renal handling and metabolism, food intake, and methodological interferences may influence the concentration of serum creatinine.2,3

In patients with paraplegia and tetraplegia determination and monitoring of kidney function is of great importance and GFR is considered to be the best marker of renal function. Creatinine correlates with muscle mass1 but in patients with spinal cord injury (SCI) muscle mass is decreasing. Creatinine clearance determinations involve 24 h urine collection and may prove the accuracy but it is often difficult and time consuming for patients to perform.4 In patients with SCI urine collection is difficult and often inaccurate and creatinine clearance is encumbered with some uncertainties.

Cystatin C is a non-glycosylated low molecular weight protein (Mr=13 359).5 It is produced by all nucleated cells at a constant rate,6 freely filtered in the glomeruli and reabsorbed and catabolized in the proximal tubular cells.7 The characteristics of cystatin C indicates that its serum concentration is mainly determined by GFR making cystatin C an endogenous parameter of GFR.7,8,9,10,11,12,13 Cystatin C is independent of gender, age (>1 year) and muscle mass.11,14,15,16

The aim of this study was to investigate the relationship between serum cystatin C, serum creatinine, and 51Cr-EDTA-clearance in patients with spinal cord injury.

Materials and methods

Patients

Thirty-one patients with SCI classified ASIA A or B (24 men and seven women) aged between 20.3 and 68.0 years (40.4±12.5 years; mean±SD) were included. Patients with heart failure, kidney disease, malignancies and incomplete SCI (ASIA C-E) were excluded.

Sixteen patients had motor complete paraplegia. Twelve of the 16 patients had traumatic injury of the thoracic spine, one traumatic injury of the lumbar spine, one congenital myelomeningocele, one an epidural abscess (cervical five to thoracic one), and one patient had a transverse myelitis. Fourteen patients had traumatic complete tetraplegia, and one patient had an epidural abscess leading to tetraplegia.

Six patients did not receive any medical treatment. Of the remaining 25 patients four were treated with trimetoprim, and one patient was treated with steroids in low doses. None of the patients were treated with salicylates, phenacemide, or cyclosporine. Several patients were treated with laxatives, muscle relaxants, and low molecular heparin. In the patients treated with trimetoprim, no differences of serum cystatin C and creatinine were found compared to patients without trimetoprim treatment.

The study protocol was approved by the local committee of ethics according to the Declaration of Helsinki and all the patients gave their written informed consent prior to the start of the study.

Procedure

During a period of 1½ years, patients arrived at the Spinal Cord Unit, Viborg-Kjellerup County Hospital. Blood samples were drawn to analyze serum cystatin C, creatinine, urea nitrogen and albumin. A 24 h urine specimen was collected for determination of creatinine clearance. 51Cr-EDTA-clearance was measured. Height and weight were registered.

Analytical methods

Blood samples were drawn from anticubital vein using Venoject VT-050UX tubes (Terumo-Europe, Leuven, Belgium). The blood samples were centrifuged at 1500 g for 15 min. Serum was isolated and analyzed on the same day, or within 2 months after storage at −80°C.

Serum cystatin C was analyzed using the Dade Behring N Latex Cystatin C assay on the Dade Behring Nephelometer II (Dade Behring Diagnostics, Marburg, Germany).17 The method was a fully automated particle-enhanced nephelometric immunoassay (PENIA). The assay time was 6 min for cystatin C compared to 5 min for creatinine and the sample volume was 40 μL serum or plasma. The cystatin C assay could be performed along with 70 other protein assays (C-reactive-protein, immunoglobulins etc) on the Dade Behring Nephelometer II. The cost of a cystatin C test was approximately £2 compared to £0.2 for a creatinine test.

Serum and urine creatinine (enzymatic method), albumin, and urea nitrogen were analyzed on the Vitros 950 clinical chemistry system (Ortho-Clinical Diagnostics, Rochester, NY, USA).

GFR was measured by a single injection technique using 51chromium-ethylene-diamine-tetra-acetate complex (51Cr-EDTA) by a multiple plasma sample method.18

Estimated GFR was calculated using the equation developed from the Modification of Diet in Renal Disease (MDRD) study.19

Creatinine-clearance and 51Cr-EDTA-clearance was adjusted to a body surface area of 1.73 m2.

Statistical analysis

Statistical analysis was performed using Student Edition of statistic version 2.0 for Windows 98 (Analytical Software, Tallahassee, FL, USA). Data are expressed as mean values±SD. Correlation analysis was performed by calculation of Pearson's correlation coefficient (non-parametric). A P-value <0.05 was considered significant. Receiver operating characteristics (ROC) plots were performed as described by Zweig et al20 using GraphROCTM for Windows version 2.0 (developed by Veli Kairisto, Turku, Finland and Allan Poola, Tallin, Estonia).

Results

Patients' characteristics are listed in Table 1. The values are given as means±SD. Serum cystatin C and serum creatinine showed an inverse relationship to 51Cr-EDTA-clearance values (Figure 1a,b).

Table 1: Patients' characteristics. All data are expressed as mean values±SD
Figure 1
Figure 1

Relationship between 51Cr-EDTA-clearance and (a) serum cystatin C and (b) serum creatinine in 31 patients. GFR80 mL/min/1.73 m2 was considered as a normal kidney function (vertical lines). (a) The upper limit of the reference interval for serum cystatin C was 1.02 mg L−1 (horizontal). (b) The upper limit of reference interval for serum creatinine was 111 μmol L−1 for males (horizontal solid line) and 95 μmol L−1 for females (horizontal dotted line). Open circles=females, solid circles=males

Serum cystatin C and serum creatinine and their relationship with 51Cr-EDTA-clearance were linearized by plotting their reciprocal values (Figure 2a,b). There was a linear relationship between 51Cr-EDTA-clearance and 1/serum cystatin C (r=0.72; P-value <0.0001), and between 51Cr-EDTA-clearance and 1/serum creatinine (r=0.26; P-value=0.16).

Figure 2
Figure 2

Relationship between 51Cr-EDTA-clearance and (a) 1/serum cystatin C, (b) 1/serum creatinine, and (c) creatinine-clearance

Creatinine clearance was measured in 23 patients and showed decreasing values with decreasing 51Cr-EDTA-clearance values (Figure 2c). There was a linear relationship between 51Cr-EDTA-clearance and creatinine clearance (r=0.59; P-value=0.003).

Non-parametric ROC plots for serum cystatin C and serum creatinine are shown in Figure 3. Comparison of the areas under the curves (AUC) for serum cystatin C (AUC=0.912; SE=0.065) and serum creatinine (AUC=0.507; SE=0.115) revealed significant difference (P-value=0.0005).

Figure 3
Figure 3

Non-parametric ROC plots for serum cystatin C (solid line) AUC=0.912 and serum creatinine (dotted line) AUC=0.507

In 23 patients creatinine clearance was determined. AUC for serum cystatin C, serum creatinine, creatinine clearance and estimated GFR values are listed in Table 2. Also, in this subgroup of patients significant difference was demonstrated between serum cystatin C and serum creatinine (P-value=0.01). No significant difference was found between serum cystatin C and creatinine clearance or estimated GFR.

Table 2: Area under the curve (AUC) and standard error (SE) calculated using ROC plots

Discussion

The findings in the present study suggest that serum cystatin C is a better marker of the renal function than serum creatinine and creatinine clearance in adult patients with SCI.

Patients with spinal cord injury are at increased risk of developing renal insufficiency from several factors, eg high detrusor leak-point pressure, vesicoureteral reflux, recurrent ascending infection and renal calculi. The patients need to have their renal function examined on a regular basis.21,22 As a routine 51Cr-EDTA-clearance, creatinine clearance and serum creatinine is used for this purpose. 51Cr-EDTA-clearance is laboratory-intensive and time consuming.23 The measured 24 h creatinine clearance is widely used but may be subject to error because of the inherent difficulty in collecting a 24 h urine specimen and laboratory error involved in measuring serum and urinary creatinine concentrations. Serum creatinine depends on the creatinine production. In patients with SCI serum creatinine is often significantly decreased as a result of the diffuse muscle atrophy which commonly accompanies muscle denervation.24 In patients with SCI normal serum creatinine values could conceal a clinically significant reduction in GFR. Therefore it would be of value to have a simple, safe test to measure renal function.

The present study, including 31 patients with spinal cord injury, indicates that, serum cystatin C is a reliable parameter of GFR. In agreement with previous studies dealing with patients having different kidney diseases25 serum cystatin C and creatinine increased with decreasing GFR values.23,25,26 Serum cystatin C correlated significantly better to GFR than serum creatinine and estimated GFR. ROC plots allow simultaneous comparison of the diagnostic specificity and sensitivity of serum cystatin C and creatinine. It can be used to evaluate the capability of serum levels of cystatin C and creatinine to correctly classify subjects into groups with normal or reduced GFR. A comparison of the area under the curves of the ROC plots for serum cystatin C and creatinine (P-values=0.0005) reveals significant difference, indicating that the diagnostic accuracy of serum cystatin C was superior to that of serum creatinine in identifying patients with reduced GFR.

Serum cystatin C levels can be determined by fully automated commercially available immunoassays which are rapid and precise.27 No major interference with haemoglobin, bilirubin, triglycerides, and rheumatoid factors have been demonstrated using the assays.17,25 Reference intervals are established and the same reference interval can probably be used in children older than 1 year and in adults independent of gender.16,28 Furthermore, cystatin C is independent of muscle mass in contrast to creatinine. To our knowledge, no drugs have been reported to influence the serum levels of cystatin C in clinical practice.

Previous studies dealing with serum cystatin C as a parameter of GFR have suggested serum cystatin C to be as good a marker as serum creatinine11,26,28 or even superior to serum creatinine.8,10 Further studies demonstrated that in patients with normal to moderate impaired kidney function serum cystatin C seemed to be superior to creatinine as a parameter of GFR.28

In conclusion, the results demonstrate that serum cystatin C may be a better marker of the GFR than serum creatinine and creatinine clearance in patients with SCI. Serum cystatin C has the advantage that its concentration is independent of gender, muscle mass and age (>1 year) making cystatin C a promising parameter of the renal function in patients with SCI.

References

  1. 1.

    , , . Serum creatinine as an index of renal function: new insights into old concepts Clin Chem 1992 38: 1933–1953

  2. 2.

    , . Nutritional therapy for the uremic patient In: Brenner BM (ed) The kidney 5th edn Philadelphia: W.B. Saunders Company 1996 pp. 2382–2423

  3. 3.

    , . Interferences in current methods for measurements of creatinine Clin Chem 1991 37: 695–700

  4. 4.

    , . Creatinine a review Clin Chem 1980 26: 1119–1126

  5. 5.

    , . Human gamma-trace, a basic microprotein: amino acid sequence and presence in the adenohypophysis Proc Natl Acad Sci USA 1982 79: 3024–3027

  6. 6.

    et al. Structure and expression of the human cystatin C gene J Biochem 1990 268: 287–294

  7. 7.

    . Diagnostic value of analysis of cystatin C and protein HC in biological fluids Clin Nephrol 1992 38: (Suppl 1) 20–27

  8. 8.

    et al. Serum cystatin C measured by automated immunoassay: a more sensitive marker of changes in GFR than serum creatinine Kidney Int 1995 47: 312–318

  9. 9.

    , . New markers for the determination of GFR: iohexol clearance and cystatin C serum concentration Kidney Int 1994 46: 17–19

  10. 10.

    , . Sandwich enzyme immunoassay of cystatin C in serum with commercially available antibodies Clin Chem 1993 39: 1885–1890

  11. 11.

    , , , . Serum cystatin C as a marker of the renal function Scand J Clin Lab Invest 1998 58: 585–592

  12. 12.

    et al. Cystatin C measurement and its practical use in patients with various renal diseases Clin Nephrol 1997 48: 104–108

  13. 13.

    , , , . Renal handling of radiolabelled human cystatin C in the rat Scand J Clin Lab Invest 1996 56: 409–414

  14. 14.

    et al. Cystatin C and creatinine after successful kidney transplantation in children Clin Nephrol 1999 52: 371–376

  15. 15.

    , , . Reference intervals for serum cystatin C and serum creatinine in adults Clin Chem Lab Med 1998 36: 393–397

  16. 16.

    et al. Reference interval for serum cystatin C in children Clin Chem 1999 45: 1856–1858

  17. 17.

    , , . Evaluation of the Dade Behring N Latex Cystatin C assay on the Dade Behring Nephelometer II System Scand J Clin Lab Invest 1999 59: 1–8

  18. 18.

    . Current status on assessment and measurement of glomerular filtration rate Clin Physiol 1985 5: 1–17

  19. 19.

    et al. A more accurate method to estimate glomerular filtration rate from serum creatinine: A new prediction equation An Int Med 1999 130: 461–471

  20. 20.

    , . Receiver-operating characteristic (ROC) plots: a fundamental evaluation tool in clinical medicine Clin Chem 1993 39: 561–577

  21. 21.

    et al. Urologic complications of spinal cord injury. In: Spine trauma Philadelphia: W.B. Saunders Compnay 1998 pp. 630–638

  22. 22.

    et al. Prediction of creatinine clearance from serum creatinine in spinal cord injury patients Paraplegia 1983 21: 23–29

  23. 23.

    et al. The evaluation of creatinine clearance in spinal cord injury patients J Urol 1986 136: 366–369

  24. 24.

    et al. Monitoring of renal function in patients with spinal cord injury BJU Int 2000 85: 1014–1018

  25. 25.

    et al. Initial evaluation of cystatin C measurement by particle-enhanced immunonephelometry on the Behring nephelometer systems (BNA, BN II) Clin Chem 1997 43: 1016–1022

  26. 26.

    , , . The blood serum concentration of cystatin C (gamma-trace) as a measure of the glomerular filtration rate Scand J Clin Lab Invest 1985 45: 97–101

  27. 27.

    et al. Serum concentration of cystatin C, factor D and β2-microglobulin as a measure of glomerular filtration rate Acta Med Scand 1985 218: 499–503

  28. 28.

    et al. Serum cystatin C as an endogenous parameter of the renal function in patients with normal to moderately impaired kidney function Clin Nephrol 2000 54: 203–209

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Acknowledgements

The study has been supported by grants from Helen and Ejnar Bjørnows Foundation. The authors are grateful to Mrs Mette Kjelst and Mrs Lilian Christensen for their technical assistance.

Author information

Affiliations

  1. Department of Internal Medicine, Viborg-Kjellerup County Hospital, Viborg, Denmark

    • SA Thomassen
    •  & E Randers
  2. Department of Spinal Cord Injury, Viborg-Kjellerup County Hospital, Viborg, Denmark

    • IL Johannesen
  3. Department of Clinical Biochemistry, Viborg-Kjellerup County Hospital, Viborg, Denmark

    • EJ Erlandsen
  4. Department of Nuclear Medicine, Viborg-Kjellerup County Hospital, Viborg, Denmark

    • J Abrahamsen

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Correspondence to SA Thomassen.

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https://doi.org/10.1038/sj.sc.3101320

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