*Geek Box: Urinary Samples in Nutritional Epidemiology
Per the Geek Box above, the key feature of urinary recovery biomarkers like sodium or potassium, is that urine is the primary means of excretion. This means that over a 24hr period, almost all sodium intake through the diet will ultimately be excreted through urine.
However, this also means that the method of collection is very important. A spot urine sample, which is a single sample collected at particular time of day [which may be random, or planned, i..e, first void in the morning], may be appealing because it is easy for investigators to collect, and does not require a lot of compliance. However, it will inevitably fail to capture 24hr intake, and will be dependent on the timing of dietary intake and hydration status at that particular time of the day, leading to substantial measurement error.
Particularly for sodium, it is important to obtain a full collection of all urine over a 24hr period. However, this is burdensome on investigators and participants. This may mean that a single 24hr collection is obtained in a study, however, the issue with this approach is that day-to-day intake in nutrients like sodium can vary substantially, and thus while it the 24hr collection may accurately reflect sodium intake that day, it only reflects that day. Thus, the variation between all participants will lead to measurement error.
To overcome these limitations, the “gold standard” means of assessing sodium [or potassium] intake is to collect multiple 24hr urinary sample collections, and validation studies have indicated that up to 6 x 24hr collections per participant provides sufficient samples to correct for variation in individual intake [within-person error], and minimise variation from individual to individual in the study [between-person variation]. In assessing any epidemiological study that measures sodium intake, these differences in urinary samples are critical to interpreting the data.