Can sodium and potassium measured in timed voids be used as reference instruments for validating self-report instruments? Results from a urine calibration study

Background: Sodium and potassium measured in 24-h urine collections are often used as reference measurements to validate self-reported dietary intake instruments. Objectives: To evaluate whether collection and analysis of a limited number of urine voids at specified times during the day (“timed voids”) can provide alternative reference measurements, and to identify their optimal number and timing. Methods: We used data from a urine calibration study among 441 adults aged 18–39 y. Participants collected each urine void in a separate container for 24 h and recorded the collection time. For the same day, they reported dietary intake using a 24-h recall. Urinary sodium and potassium were analyzed in a 24-h composite sample and in 4 timed voids (morning, afternoon, evening, and overnight). Linear regression models were used to develop equations predicting log-transformed 24-h urinary sodium or potassium levels using each of the 4 single timed voids, 6 pairs, and 4 triples. The equations also included age, sex, race, BMI (kg/m²), and log creatinine. Optimal combinations minimizing the mean squared prediction error were selected, and the observed and predicted 24-h levels were then used as reference measures to estimate the group bias and attenuation factors of the 24-h dietary recall. These estimates were compared. Results: Optimal combinations found were as follows: single voids—evening; paired voids—afternoon + overnight (sodium) and morning + evening (potassium); and triple voids—morning + evening + overnight (sodium) and morning + afternoon + evening (potassium). Predicted 24-h urinary levels estimated 24-h recall group biases and attenuation factors without apparent bias, but with less precision than observed 24-h urinary levels. To recover lost precision, it was estimated that sample sizes need to be increased by ∼2.6–2.7 times for a single void, 1.7–2.1 times for paired voids, and 1.5–1.6 times for triple voids. Conclusions: Our results provide the basis for further development of new reference biomarkers based on timed voids. Clinical Trial Registry: clinicaltrials.gov as NCT01631240.