Table 2.

Within- and among-subject variability in phosphorus and creatinine measures

Subject24-h Urine Phosphorus, mg/da24-h Urine Creatinine, mg/da24-h Urine Phosphorus-to-Creatinine, mg/mgaPhosphorus Balance, mg/dbNet Phosphorus Absorption, mg/db%Net Phosphorus AbsorptionbSerum Phosphorus, mg/dlcSerum Creatinine, mg/dlceGFR, ml/mincSerum iPTH, pg/mlSerum iFGF23, pg/ml
Within Subjects
 Subject 1991 (248)1911 (168)0.52 (0.12)−260731473.8 (0.3)3.0 (0.1)29 (0.6)143 (55)74 (12)
25%9%23%8.4%2.3%2%39%16%
 Subject 2682 (105)1023 (94)0.67 (0.08)−229452293.8 (0.4)1.9 (0.1)34 (3)47 (10)101 (6)
15%9%12%9.6%7.4%10%27%5%
 Subject 31083 (209)1277 (140)0.84 (0.10)−231845544.0 (0.5)1.5 (0.1)37 (2)60 (10)135 (8)
19%11%12%13.1%5.0%7%15%18%
 Subject 4843 (127)1180 (135)0.71 (0.10)82925594.0 (0.5)1.7 (0.2)31 (4)72 (9)62 (28)
17%11%13%13.4%10.0%11%14%50%
 Subject 5693 (243)1555 (182)0.44 (0.14)181872533.6 (0.5)1.7 (0.05)41 (1)59 (4)50 (23)
35%12%32%14.5%2.9%2%10%31%
 Subject 6535 (158)771 (215)0.69 (0.26)344901633.2 (0.5)1.7 (0.1)41 (3)51 (11)74 (14)
30%28%38%15.2%7.0%7%22%20%
 Subject 7708 (241)1691 (337)0.43 (0.14)203923603.7 (1.3)3.4 (0.9)26 (9)64 (10)62 (26)
34%20%32%34.1%25.9%37%23%50%
 Subject 8225 (136)731 (123)0.31 (0.15)680903603.8 (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)
 %CV3734313432021733164736
  • iPTH, intact parathyroid hormone; iFGF23, intact fibroblast growth factor 23; %CV, percent coefficient of variation.

  • a 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.

  • b 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.

  • c 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).