Different equations to combine creatinine and cystatin C to predict GFR. Arithmetic mean of existing equations performs as well as complex combinations

Abstract

Purpose: To test various ways of combining creatinine and cystatin C in equations to predict glomerular filtration rate (GFR). Material and methods: Performance of the following expressions to predict GFR was compared with measured GFR (iohexol clearance, mL/min/1.73 m²) in 857 patients: (i) Lund-Malmö creatinine equation, (ii) Grubb cystatin C equation, (iii) arithmetic mean of (1) and (2), (iv) geometric mean of (1) and (2), (v) linear regression on (1) and (2), (vi) regression on (1) and cystatin C, and (vii) regression on creatinine, cystatin C, age and gender. Results: For the entire cohort median percent error (bias) was <5% for all expressions, though all expressions tended to underestimate (−8.3 to 15.8%) GFR at levels ≥90 mL/min/1.73 m². The five expressions combining creatinine and cystatin C significantly improved correlation and accuracy (p<0.001) within 15 and 30% of measured GFR compared with the equations based on the separate analytes and with no significant difference between the five expressions. In a subgroup of patients with neurological disease and muscle atrophy the cystatin C equation performed better than the expressions combining creatinine and cystatin C. Conclusion: Simply calculating the arithmetic mean of predicted GFR based on separate creatinine and cystatin C equations performs equally well as more complex equations. Reporting GFR based on separate creatinine and cystatin C equations, and their arithmetic mean also has the definite advantage that the physician can choose the estimated GFR, most appropriate depending on the clinical setting and patient characteristics.

Keywords::

• Kidney disease
• kidney function tests
• glomerular filtration rate
• renal insufficiency

Acknowledgments

We are grateful to librarian Elisabeth Sassersson for excellent service regarding the literature references.

This study was supported by the Swedish Science Research Council (Project no 5196), the Medical Faculty in Lund, Region Skåne and by grants from A. Österlund's, and G. and J. Kock's Foundations.

Declaration of interest: The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.

Appendix

In the equations predicting glomerular filtration rate (GFR) plasma creatinine (pCr) is expressed in μmol/L (to convert pCr in μmol/L to mg/dL, divide by 88.4), plasma cystatin C in mg/L, and age in years. Estimated GFR is expressed in mL/min/1.73 m² body surface area.

Lund-Malmö creatinine equation without body weight measure (16)

GFR = e^{X−0.0124*age+0.339*ln(age)−0.226 (if female)}

X = 4.62−0.0112*pCr (if pCr<150 μmol/L)

X = 8.17+0.0005*pCr−1.07*ln(pCr) (if pCr≥150 μmol/L)

Grubb cystatin C equation based solely on adults (6)

GFR = 86.49*pCyC^−1.686*0.948 (if female) = e^{4.46−1.686*ln(pCyC)−0.053 (if female)}

Regression on Lund-Malmö creatinine (Eq1) and Grubb cystatin C (Eq2) equations

GFR = Eq1^0.567*Eq2^0.440

Regression on Lund-Malmö creatinine equation (Eq1) and cystatin C

GFR = e^1.928*Eq1^0.574*pCyC^−0.749

Regression on creatinine, cystatin C, age and gender

GFR = e^{X−0.718*In(pCyC)-0.00452*age+0.0549*In(age)-0.153 (if female)}

X = 5.02−0.00716*pCr (if pCr<150 μmol/L)

X = 7.07+0.0004*pCr−0.636*ln(pCr) (if pCr≥150 μmol/L)