Calciuria in symptom-free primigravid women remote from term: Is the response to an oral calcium challenge predictable?

OBJECTIVE: This study was undertaken to compare the calciuric response in s ymptom-free primigravid women to an oral calcium load between those with no rmal urinary calcium excretions and those with relatively low urinary calci um excretions. STUDY DESIGN: This was a prospective clinical trial. Eligible primigravid w omen between 16 and 20 weeks' gestation provided a 24-hour urine sample for determination of urinary calcium/urinary creatinine ratio. On the basis of these results the patients were divided into 2 groups: a relatively hypoca lciuric group, in which the urinary calcium excretion was less than or equa l to 3.4 mg.kg(-1).24 h(-1), and a normocalciuric group, in which the urina ry calcium excretion was >3.4 mg kg(-1).24 h(-1). All participants undertoo k a 3-day low calcium dietary regimen. On the fourth day women underwent an oral calcium challenge. A 2-hour urine sample was collected before ingesti on of 1 g calcium carbonate (preload). One hour after ingestion the women a gain collected a 2-hour urine sample (postload). The urinary calcium/urinar y creatinine ratios in the preload and postload samples were determined and compared within and between the groups. RESULTS: The mean change (+/-SD) between the preload and postload urinary c alcium/urinary creatinine ratios in the relatively hypocalciuric group was 0.60 +/- 1.44 (P=.04); that in the normocalciuric group was 3.09 +/- 2.26 ( P=.11). There was a 5-fold difference in the response to calcium load betwe en the hypocalciuric women and the normocalciuric women (0.60 vs 3.09), alt hough this difference was not statistically significant (P=.20). CONCLUSIONS: Both hypocalciuric and normocalciuric women responded to an or al calcium challenge by an increase in the calcium excretion. The cause of the hypocalciuria in women at increased risk for preeclampsia is therefore not simply poor absorption of calcium.