Background: The present study was designed to determine the reliability of the current practice of random pre-dialysis phosphate testing in subjects receiving hemodialysis therapy, since phosphate levels exhibit a significant variation in a 24-hour period under usual physiologic conditions.
Methods: Subjects receiving chronic hemodialysis (HD) were invited to participate during an incidental hospitalization. In Study A subjects (n=31) had serum phosphate tested three times on a single non-dialysis day, between 6 to 7 am (A1), between 11 am to 12:30 pm (A2), and 3:45 pm to 4:45 pm (A3). In study B subjects (n=25) had serum phosphate tested just before (B1) and 2 hours after lunch (B2), on a non-dialysis day. For Study A the collection times coincided with the start times of the out-patient dialysis shifts at our institution. All patients continued their usual phosphate binder therapy, if any. For study A the results were analyzed using one-way repeated measures analysis of variance or Friedman repeated measures analysis of variance on ranks, as opposite. Paired t-test was used for Study B. Results are expressed as mean+/- standard deviation.
Results: Twenty-three men and eight women (mean age 62.2+/-15.2 years) were enrolled in Study A (24 Caucasian, 7 African-American). Nineteen men and six women (mean age 65.5+/-12.8 years) participated in study B (19 Caucasian, 6 African-American). In Study A, there was no significant difference in the mean serum phosphate levels in the samples collected through the day A1 (4.5+/-1.3 mg/dL), A2 (4.5+/-1.3 mg/dL) and A3 (4.7+/-1.5 mg/dL) (p=0.19 for comparison of the three). The mean amplitude of circadian variation (peak minus trough) was 0.64+/- 0.37 mg/dL. Similarly, there was no significant difference in the mean serum phosphate before (4.4+/-1.4 mg/dL) and two hours after lunch (4.4+/-1.5 mg/dL) (p =0.6). The mean of the difference in serum phosphate (post-prandial minus pre-prandial) was 0.2+/-0.4 mg/dL.
Conclusion: Our results in hospitalized HD subjects indicate there is no significant difference in phosphate levels at different times of a single day or in relation to meals.