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Published on: May 10, 2013
Estimated GFR Accuracy When Cystatin C- and Creatinine-Based Estimates Are Discrepant in Older Adults
O Alison Potok1,2, Dena E Rifkin1,2, Joachim H Ix1,2
1Division of Nephrology-Hypertension, University of California San Diego, San Diego, CA.
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 m2 (lower eGFRcys), within 5 mL/min/1.73 m2 (reference), or ≥ 5 mL/min/1.73 m2 (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 m2, 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.
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