. Author manuscript; available in PMC: 2017 May 1.

Published in final edited form as: Kidney Int. 2016 Jan 21;89(5):1099–1110. doi: 10.1016/j.kint.2015.10.011

Table 3.

Residual Kidney Function Study CL_UREA and CL_{UREA, CREAT} Estimating Equations’ Performance in 826 Patients from the Validation Cohort (NECOSAD)

Equation	Markers	Other Variables	RMSE	Median Bias (95%CI)²^, ³	Precision (95% CI)⁴^, ⁵	Accuracy (95% CI)⁶^, ⁷
				Measured Clearance – Estimates Clearance	IQR of Bias	Estimated within ± 2 mL/min of CL_UREA
CL_UREA mL/min				mL/min	mL/min	%
RKF Study	Urea, Creatinine		0.753	0.2 (0.01, 0.3)	2.2 (2.1, 2.4)	75 (72, 78)
RKF Study	BTP	Sex	0.612	0.4 (0.3, 0.5)^a	1.8 (1.6, 2.0)^b	81 (78, 83) ^b^***
RKF Study	B2M	Sex	0.584	0.7 (0.6, 0.8)^a	1.6 (1.5, 1.7) ^b	79 (76, 81) ^b^*
RKF Study	Cystatin C		0.667	0.5 (0.4, 0.6)^a	2.0 (1.8, 2.1)	79 (76, 82) ^b^***
RKF Study	BTP, B2M	Sex	0.569	0.5 (0.4, 0.6)^a	1.5 (1.4, 1.7) ^b	81 (79, 84) ^b^***
CL_{UREA, CREAT}, mL/min/1.73 m²				mL/min/1.73 m²	mL/min/1.73 m²	Estimates within ± 2 mL/min/1.73 m² of CL_{UREA, CREAT}, %
RKF Study Equations
RKF Study	Urea, Creatinine		0.669	0.3 (0.2, 0.5)^a	2.7 (2.5, 2.9)	68 (65, 72)
RKF Study	BTP	Sex	0.556	0.6 (0.5, 0.7)^a	2.1 (2.0, 2.3)	71 (68, 74)
RKF Study	B2M	Sex	0.553	1.0 (0.9, 1.1)^a	1.9 (1.7, 2.1)	69 (66, 72)
RKF Study	Cystatin C		0.605	0.7 (0.5, 0.9)^a	2.3 (2.1, 2.5)	72 (69, 75) ^b^*
RKF Study	BTP, B2M	Sex	0.506	0.7 (0.6, 0.8)^a	1.8 (1.6, 1.9)	75 (72, 78) ^b^***

Abbreviations: CL_UREA, urinary urea clearance (mL/min); CL_{UREA, CREAT}, average of urinary urea and creatinine clearance (mL/min/1.73 m²); BTP, β-trace protein; B2M, β2 microglobulin; IQR, Interquartile Range; RMSE, root mean squared error; NECOSAD, The Netherlands Cooperative Study on the Adequacy of Dialysis; RKF, Residual Kidney Function

Note: In the NECOSAD Study, 826 participants had 989 clearance measurements. Performance data are only for the first measurement (n=826).

Root mean squared error (RMSE) from linear regression of natural log transformed measured urinary clearance on natural log transformed estimated urinary clearance. A smaller RMSE implies a better model fit.

Bias is defined as the median difference between measured clearance and estimated clearance. Confidence intervals are calculated by bootstrapping with 2000 replicates.

Significance of bias in estimation using BTP, B2M and cystatin C equations compared with urea+creatinine equation was determined by Wilcoxon matched-pairs signed-ranks test (a and b indicate worse and better than the urea+creatinine equation, respectively). All p-values are <0.001.

⁴

Precision is defined as interquartile range of the median bias. Confidence intervals are calculated by bootstrapping with 2000 replicates.

⁵

Significance of the precision of estimation using BTP, B2M and cystatin C equations compared with urea+creatinine equation was determined by interquartile range regression with bootstrapped (2000 replicates) standard errors (a and b indicate worse and better than the urea+creatinine equation, respectively). All significant differences have p-values <0.001.

⁶

Accuracy is defined as estimates within ± 2 mL/min of CL_UREA or ± 2.5 mL/min/1.73 m² of CL_{UREA, CREAT}. Confidence intervals are calculated by bootstrapping with 2000 replicates.

⁷

Significance of the accuracy of estimation using BTP, B2M and cystatin C equations compared with urea+creatinine equation was determined by McNemar’s chi-squared test. (a and b indicate worse and better than the urea+creatinine equation, respectively).

p<0.05;

^**

p<0.01;

^***

p<0.001