Adjusting urinary chemical biomarkers for hydration status during pregnancy
- DOI
- Language of the publication
- English
- Date
- 2018-06-08
- Type
- Article
- Author(s)
- MacPherson, Susan
- Arbuckle, Tye E.
- Fisher, Mandy
- Publisher
- Springer Nature
Abstract
One way of assessing a population's exposure to environmental chemicals is by measuring urinary biomarker concentrations, which can vary depending on the hydration status of the individual. The physiological changes that occur during pregnancy can impact the hydration adjustment approaches, such as calculating the individual's urinary flow rate (UFR), or adjusting concentrations using specific gravity (SG) or creatinine. A total of 1260 serial spot urine samples were collected from 80 women, averaging 32.4 years of age, throughout and shortly after pregnancy. The relationship between each approach was examined and time of day and across pregnancy differences were tested using linear mixed models. The correlation between the calculated excretion rate and each of the adjustment techniques was examined on a selection of seven phthalate metabolites. Based on the linear mixed model results, we found that UFR and creatinine excretion rates differed systematically across the population, with respect to body mass index (BMI) and time. SG differed with respect to BMI, but there were no systematic time trends. SG had the highest within-person reproducibility, according to the intraclass correlation coefficient (ICC). The excretion rate of each of the phthalates was most strongly correlated with the SG-standardized concentration. This analysis showed that SG showed a slightly better within-person reproducibility and the least amount of systematic variation when compared to creatinine adjustment. Therefore, SG correction appears to be a favorable approach for correcting for the hydration status of the pregnant women from this cohort.
Plain language summary
Health Canada is responsible for protecting the health of Canadians by assessing and managing the health risks associated with exposure to environmental contaminants. To assess a population’s exposure to a given contaminant, chemical concentrations are often measured in urine samples. The more fluids that are ingested, the more dilute the urine and the lower the concentration of the chemical that will be measured in the urine. Different people will have different fluid intakes, so to directly compare concentrations of a chemical between people, it is important to adjust the concentration of a chemical in their urine by a measure of how dilute their urine is. Due to the physical changes that can affect the dilution of urine samples during pregnancy, this study was designed to evaluate the adjustment methods and determine the best approach to account for the dilution of urine samples collected during pregnancy. An accurate method should remain constant within and between individuals. These methods include calculating the individual’s urinary flow rate by dividing the total volume of the urine sample by the time since the previous void, or adjusting concentrations using urinary specific gravity (SG) or creatinine concentration. Overall, urinary flow rates and creatinine concentration differed across the pregnant population, with respect to body mass index and time of day when the urine was collected, while specific gravity showed no trend within a day or across pregnancy. In addition, specific gravity also has the lowest within person variability. The outcome of this analysis suggests that specific gravity adjustment is the best approach for correcting urinary biomarker concentrations for dilution in pregnant women. This study has provided further understanding into the physiological and time-based factors affecting urinary dilution and may better inform human health risk assessment with respect to the pregnant population.
Subject
- Health,
- Health and safety