Estimated GFR Accuracy When Cystatin C– and Creatinine-Based Estimates Are Discrepant in Older Adults

RATIONALE & OBJECTIVE: Serum creatinine and cystatin C are used to estimate glomerular filtration rate, but creatinine-based estimated glomerular filtration rate (eGFRcr), cystatin C–based estimated glomerular filtration rate (eGFRcys), and combined creatinine- and cystatin C–based estimated glomerular filtration rate (eGFRcr-cys) are often divergent, particularly in older adults. We investigated which estimated glomerular filtration rate (eGFR) was more accurate and less biased compared with measured glomerular filtration rate (mGFR). STUDY DESIGN: A diagnostic test study from the Berlin Initiative Study. SETTING & PARTICIPANTS: The study population included 657 individuals aged 70 years or older with iohexol plasma clearance (mGFR) and serum creatinine and cystatin C measurements: 567 community-dwelling participants and 90 with a serum creatinine of ≥1.5 mg/dL. TESTS COMPARED: We defined 3 groups on the basis of the difference eGFRcys − eGFRcr: whether < −5 mL/min/1.73 m(2) (lower eGFRcys), within 5 mL/min/1.73 m(2) (reference), or ≥ 5 mL/min/1.73 m(2) (lower eGFRcr). eGFRcr, eGFRcys, and eGFRcr-cys were compared to mGFR to assess bias and accuracy. OUTCOME: Median bias (eGFR minus mGFR) with 95% CIs and accuracy (percentage of eGFR values within ±30% of mGFR). RESULTS: The mean ± standard deviation age was 78 ± 6 years; the mean eGFRcys, eGFRcr, and eGFRcr-cys were 59 ± 23, 64 ± 20, and 61 ± 22 mL/min/1.73 m(2), respectively, and the mean mGFR was 56 ± 19 mL/min. Half of the participants were in the lower eGFRcys group (n=337, 51%). Among them, the median bias for eGFRcys was the lowest (median bias, −2.7; 95% CI, −3.8 to −1.9) compared with the other eGFR equations. Conversely, in the lower eGFRcr group (n=121, 18%), the median bias for eGFRcr was the lowest compared with those for eGFRcys and eGFRcr-cys (2.9; [95% CI, 0.9-4.8] vs 13.8 [95% CI, 11.4-15.6] and 9.5 [95% CI, 7.7-11.0], respectively). Accuracy (percentage of eGFR values within ±30% of mGFR) was 93% for eGFRcr in the lower eGFRcr group and 92% for eGFRcys and 94% for eGFRcr-cys in the lower eGFRcys group. LIMITATIONS: Untested generalizability in younger populations. CONCLUSIONS: Among older adults, the lower eGFR between eGFRcys and eGFRcr was a more accurate and less biased estimate of mGFR when comparing the groups.