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Abstract: SA-PO190

Relative Contributions of Excretion (EP) and Reabsorption (TRP) of Phosphate to Fractional Excretion of Phosphate (FEP) in CKD

Session Information

Category: Bone and Mineral Metabolism

  • 402 Bone and Mineral Metabolism: Clinical

Authors

  • Gosmanova, Elvira O., Albany Stratton VA Medical Center Albany, Albany, New York, United States
  • Gemoets, Darren E., Albany Stratton VA Medical Center Albany, Albany, New York, United States
  • Phelps, Kenneth R., Albany Stratton VA Medical Center Albany, Albany, New York, United States
Background

In a steady state, net flux of phosphate (P) into plasma determines EP. FEP is commonly used to depict the tubular reabsorption rate of P (TRP). EP/Ccr and TRP/Ccr, the amounts of P excreted and reabsorbed per volume of filtrate, determine both FEP and the serum P concentration (Ps) (Phelps, et al. Clin Nephrol 2015;83:167-76). However, the relative effects of EP/Ccr and TRP/Ccr on FEP have not been studied. We analyzed relationships among FEP, EP/Ccr and TRP/Ccr in CKD stages G1-G5 (dialysis excluded).

Methods

This was a retrospective study of 387 veterans seen in the nephrology clinic of the Albany VAMC between 1/2020 and 9/2021. CKD stages were based on eGFR (2012 CKD-EPI). There were 687 concurrent determinations of serum and urine P and creatinine (cr), PTH, and eGFR. EP/Ccr was calculated as Pu*crs/cru, TRP/Ccr as Ps–EP/Ccr and FEP as Pu*crs/cru*Ps or 1/{1+(TRP/Ccr)/(EP/Ccr)} (both formulas yield the same values). Relationships among variables were examined with linear regression.

Results

Measured and calculated values are shown in Table 1. EP/Ccr and FEP increased from CKD G1 to G5 by 380% and 226% but TRP/Ccr fell by only 15%. FEP correlated with EP/Ccr (R2=0.66) and less strongly with eGFR (R2=0.44), PTH (R2=0.28), and TRP/Ccr (R2=0.20) (all p<0.001). FEP was robustly determined by (TRP/Ccr)/(EP/Ccr). FEP was >20% when (TRP/Ccr)/(EP/Ccr) was <4 regardless of individual TRP/Ccr or EP/Ccr values. In 27% of cases with FEP >20%, TRP/Ccr exceeded mean values seen in CKD G1-2, and high FEP was due solely to increased EP/Ccr.

Conclusion

FEP is a function of both EP/Ccr and TRP/Ccr. At reduced GFR, EP/Ccr has a stronger effect on FEP than TRP/Ccr and PTH (a regulator of P reabsorption) have. EP/Ccr rises as GFR falls if influx of P does not fall proportionately. If EP/Ccr is sufficiently increased, FEP can be > 20% even when TRP/Ccr is ≥ normal. FEP is an inaccurate and sometimes misleading marker of P reabsorption in CKD.

Measured and calculated valuesTotal
N=687
CKD G1-2
N=114
CKD G3a
N=140
CKD G3b
N=261
CKD G4
N=146
CKD G5
N=26
crs, mean (SD), mg/dl1.9 (0.9)1.0 (0.2)1.4 (0.2)1.8 (0.3)2.7 (0.7)4.7 (1.1)
eGFR, mean (SD), ml/min/1.73m243.2 (19.5)76.6 (14.1)51.5 (6.1)37.5 (5.3)24.0 (5.6)11.7 (2.8)
Ps, mean (SD), mg/dl3.6 (0.7)3.4 (0.6)3.5 (0.6)3.5 (0.6)3.9 (0.7)4.9 (1.6)
EP/Ccr, mean (SD), mg/dl0.9 (0.6)0.5 (0.2)0.6 (0.3)0.8 (0.4)1.3 (0.6)2.4 (1.1)
TRP/Ccr, mean (SD), mg/dl2.7 (0.6)2.9 (0.5)2.8 (0.6)2.6 (0.6)2.7 (0.6)2.4 (0.7)
FEP, mean (SD), %24 (12)15 (7)18 (7)24 (9)32 (11)48 (12)
PTH, mean (SD), pg/ml96 (93)55 (30)64 (39)88 (50)143 (106)314 (286)

Funding

  • Veterans Affairs Support