Acute Changes in 24 Hour Lithogenic Urine Measures Intra and Peri Partum
Eileen Brandes, MD, Zita Ficko, MD, Elizabeth Johnson, MD, Vernon Pais, Jr., MD.
Dartmouth-Hitchcock Medical Center, Lebanon, NH.
Background: Urinary lithogenic changes during pregnancy have been hypothesized to contribute to both stone formation during pregnancy as well as long term increased stone prevalence in multigravid women. However, while short-term lithogenic changes during pregnancy have been postulated, such changes have never been demonstrated in a prospective fashion controlling for diet. Wide inter-person variation in food consumption can account for dramatic differences in 24 hour urinalyses and thus introduces potential confounding. We thus sought to define intrapartum 24 hour urine values and to assess the acute short term changes after completion of the pregnancy >= 6 weeks post-partum. To address potential dietary confounding and thus elucidate those changes intrinsic to the physiology of pregnancy, we utilized a standardized formula to control for dietary intake.
Methods: IRB approval was obtained for this prospective study. Women with singleton pregnancy presenting to the obstetrical department were offered participation. Those with gestational diabetes were excluded. Metabolic needs were assessed by an obstetrical dietitian and standardized diet was calculated based on each woman's metabolic requirements. The standardized diet consisted of boost plus, boost breeze and unlimited water. Vitamin supplements were held. Collections were obtained during the third trimester and follow-up urine was collected at 6 weeks or greater postpartum. Subjects remained on the controlled diet for 48 hours – the 24 hours preceding urine collection and continued for the 24 hours of the urine collection. Statistical analysis were performed in STATA using T-test.
Results: Of the twenty participants currently enrolled in this study, twelve have submitted a pre- and post-partum 24 hour urine collection. Post-partum collections analyzed in this study were done at any point after delivery, regardless of breast feeding and menstrual status. It was noted that there was no significant difference in known dietary-related factors of urinary volume, sodium, sulfate and urea nitrogen. Urinary calcium was found to be 302.5 pre-partum and 125.2 post-partum (p=0.001). Urinary pH was 6.59 pre-partum and 5.93 post-partum (p < 0.001). Urinary super saturation of calcium phosphate (ssCaP) was 2.1 pre-partum and 0.7 post-partum (p < 0.0001).
Conclusions: Hypercalciuria and alkaline urine are observed acutely during pregnancy. By implementing a standardized formula diet, for the first time, we have demonstrated these changes are independent of self-selected diet and thus intrinsic to pregnancy. Abnormally elevated urinary super saturation of calcium phosphate during pregnancy further suggests that these changes may have clinical importance.
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