Weaver et al.1 recently described equations for estimating the median urinary albumin-to-creatinine ratio (ACR) from the protein-to-creatinine ratio (PCR) on the basis of median regression models for log ACR in a Canadian population–based cohort. Patients with kidney transplants (KTx) were excluded. The method is proposed, for example, for answering scientific questions where ACR is required and only measured PCR is available.2
Because this method is also of interest to the transplant community, we aimed to investigate the suitability of the equations in patients with KTx and a living kidney donor (LD) cohort. Therefore, we retrospectively identified 16,990 same-day/same-person pairs of ACR and PCR measurements in patients after KTx at our transplant center in Münster, Germany and 5304 same-day/same-person pairs of ACR and PCR measurements in the LD cohort.
To estimate the median ACR of same-day measurements of PCR, we applied the following equations published by Weaver et al.1 (for all equations, the natural logarithm with base e is taken):;)
We observed that in patients with KTx, the absolute deviation between measured ACR and estimated median ACR steadily increases with increasing PCR. Ultimately, only few ACR values can be estimated within an acceptable range. Especially for PCR values >100 mg/g, the deviations seem too considerable for a valid application (Supplemental Figure 1). Furthermore, the predicted median ACR underestimates the measured ACR in the range below 100 mg/g, whereas the ACR at PCR>100 mg/g is predominantly overestimated (Supplemental Figure 2). For the estimates in KTx recipients, we calculated a root mean square deviation of 502.57 and a pseudo-R2 of 0.74. For the ACR estimation in our LD cohort, the results were comparable (Supplemental Figures 3 and 4). The root mean square deviation was 45.90, and the pseudo-R2 was 0.44 for the estimates. As pointed out by Weaver et al.,1 patient characteristics including age, sex, GFR, etc., can affect the ACR/PCR relationship. In line with this, we speculate that a (functional) single kidney is such an effector whose influence is not considered by the developed equations, knowing that many other factors may be relevant in patients with KTx.3 However, the basic relationships between ACR, PCR, and outcomes are preserved after KTx but need further investigation after living donation.4 Because no conclusions can be drawn from these observations alone, further investigation is encouraged.
Disclosures
All authors have nothing to disclose.
Funding
None.
Supplemental Material
This article contains the following supplemental material online at http://jasn.asnjournals.org/lookup/suppl/doi:10.1681/ASN.2020050545/-/DCSupplemental.
Supplemental Figure 1. In KTx recipients, the absolute deviation between measured ACR and estimated median ACR increases with rising PCR.
Supplemental Figure 2. After KTx, the prediction of the median ACR predominantly underestimates the measured ACR at values <100 mg/g and frequently overestimates it at higher values.
Supplemental Figure 3. As observed in KTx recipients, the absolute deviation between measured ACR and estimated median ACR increases with increasing PCR in the LD cohort.
Supplemental Figure 4. Comparable with the KTx data, the prediction of the median ACR in the LD cohort predominantly underestimates the measured ACR at values <100 mg/g and overestimates it at higher values.
References
1. Weaver RG, James MT, Ravani P, Weaver CGW, Lamb EJ, Tonelli M, et al.: Estimating urine albumin-to-creatinine ratio from protein-to-creatinine ratio: Development of equations using same-day measurements. J Am Soc Nephrol 31: 591–601, 2020 32024663
2. Kamińska J, Dymicka-Piekarska V, Tomaszewska J, Matowicka-Karna J, Koper-Lenkiewicz OM: Diagnostic utility of protein to creatinine ratio (P/C ratio) in spot urine sample within routine clinical practice. Crit Rev Clin Lab Sci 2020: 1–20, 2020 32058809
3. Srivastava T, Hariharan S, Alon US, McCarthy ET, Sharma R, El-Meanawy A, et al.: Hyperfiltration-mediated injury in the remaining kidney of a transplant donor. Transplantation 102: 1624–1635, 2018 29847501
4. Weiner DE, Park M, Tighiouart H, Joseph AA, Carpenter MA, Goyal N, et al.:
Albuminuria and allograft failure, cardiovascular disease events, and all-cause death in stable kidney transplant recipients: A cohort analysis of the FAVORIT trial. Am J Kidney Dis 73: 51–61, 2019 30037726
