Table 2.
Within- and among-subject variability in phosphorus and creatinine measures
| Subject | 24-h Urine Phosphorus, mg/da | 24-h Urine Creatinine, mg/da | 24-h Urine Phosphorus-to-Creatinine, mg/mga | Phosphorus Balance, mg/db | Net Phosphorus Absorption, mg/db | %Net Phosphorus Absorptionb | Serum Phosphorus, mg/dlc | Serum Creatinine, mg/dlc | eGFR, ml/minc | Serum iPTH, pg/ml | Serum iFGF23, pg/ml |
|---|---|---|---|---|---|---|---|---|---|---|---|
| Within Subjects | |||||||||||
| Subject 1 | 991 (248) | 1911 (168) | 0.52 (0.12) | −260 | 731 | 47 | 3.8 (0.3) | 3.0 (0.1) | 29 (0.6) | 143 (55) | 74 (12) |
| 25% | 9% | 23% | 8.4% | 2.3% | 2% | 39% | 16% | ||||
| Subject 2 | 682 (105) | 1023 (94) | 0.67 (0.08) | −229 | 452 | 29 | 3.8 (0.4) | 1.9 (0.1) | 34 (3) | 47 (10) | 101 (6) |
| 15% | 9% | 12% | 9.6% | 7.4% | 10% | 27% | 5% | ||||
| Subject 3 | 1083 (209) | 1277 (140) | 0.84 (0.10) | −231 | 845 | 54 | 4.0 (0.5) | 1.5 (0.1) | 37 (2) | 60 (10) | 135 (8) |
| 19% | 11% | 12% | 13.1% | 5.0% | 7% | 15% | 18% | ||||
| Subject 4 | 843 (127) | 1180 (135) | 0.71 (0.10) | 82 | 925 | 59 | 4.0 (0.5) | 1.7 (0.2) | 31 (4) | 72 (9) | 62 (28) |
| 17% | 11% | 13% | 13.4% | 10.0% | 11% | 14% | 50% | ||||
| Subject 5 | 693 (243) | 1555 (182) | 0.44 (0.14) | 181 | 872 | 53 | 3.6 (0.5) | 1.7 (0.05) | 41 (1) | 59 (4) | 50 (23) |
| 35% | 12% | 32% | 14.5% | 2.9% | 2% | 10% | 31% | ||||
| Subject 6 | 535 (158) | 771 (215) | 0.69 (0.26) | 344 | 901 | 63 | 3.2 (0.5) | 1.7 (0.1) | 41 (3) | 51 (11) | 74 (14) |
| 30% | 28% | 38% | 15.2% | 7.0% | 7% | 22% | 20% | ||||
| Subject 7 | 708 (241) | 1691 (337) | 0.43 (0.14) | 203 | 923 | 60 | 3.7 (1.3) | 3.4 (0.9) | 26 (9) | 64 (10) | 62 (26) |
| 34% | 20% | 32% | 34.1% | 25.9% | 37% | 23% | 50% | ||||
| Subject 8 | 225 (136) | 731 (123) | 0.31 (0.15) | 680 | 903 | 60 | 3.8 (0.3) | 1.9 (0.1) | 30 (1) | 44 (9) | 61 (15) |
| 60% | 17% | 51% | 6.8% | 3.2% | 5% | 30% | 36% | ||||
| Among subjects | |||||||||||
| Mean (SD) | 720 (268) | 1268 (427) | 0.58 (0.18) | 96 (330) | 819 (161) | 53 (11) | 3.7 (0.3) | 2.1 (0.7) | 34 (5) | 67 (32) | 77 (28) |
| %CV | 37 | 34 | 31 | 343 | 20 | 21 | 7 | 33 | 16 | 47 | 36 |
iPTH, intact parathyroid hormone; iFGF23, intact fibroblast growth factor 23; %CV, percent coefficient of variation.
Values for each subject are mean (SD) and %CV of 13 days of values in that individual for 24-hour urine phosphorus, 24-hour urine creatinine, 24-hour urine phosphorus-to-creatinine, and predicted phosphorus intake from daily 24-hour urine phosphorus values.
Single values of phosphorus balance, net phosphorus absorption, and %net phosphorus absorption are presented for each subject, calculated using data from the entire balance period.
Values for serum phosphorus, creatinine, eGFR, iPTH, and iFGF23 are mean (SD) and %CV of four values in that individual taken at baseline, end of the 1 week equilibration period to the diet, and at the end of each week of the balance period (i.e., four values over 3 weeks, in 1-week intervals). eGFR was calculated with the Modification of Diet in Renal Disease study equation (22).