Salt intake assessed by 24 h urinary sodium excretion in a random and opportunistic sample in Australia

OBJECTIVE: The gold standard method for measuring population sodium intake is based on a 24 h urine collection carried out in a random population sample. However, because participant burden is high, response rates are typically low with less than one in four agreeing to provide specimens. At this low level of response it is possible that simply asking for volunteers would produce the same results. SETTING: Lithgow, New South Wales, Australia. PARTICIPANTS: We randomly selected 2152 adults and obtained usable 24 h urine samples from 306 (response rate 16%). Specimens were also collected from a further 113 volunteers. Estimated salt consumption and the costs for each strategy were compared. RESULTS: The characteristics of the ‘random’ and ‘volunteer’ samples were moderately different in mean age 58 (SD 14.6 vs 49(17.7) years, respectively; p<0.001) as well as self-reported alcohol use, tobacco use, history of hypertension and prescription drug use (all p<0.04). Overall crude mean 24 h urinary salt excretion was 8.9(3.6) g/day in the random sample vs 8.5(3.3) g/day for the volunteers (p=0.42). Corresponding age-adjusted and sex-adjusted estimates were 9.2(3.3) and 8.8(3.4) g/day (p=0.29). Estimates for men 10.3(3.8) vs 9.6(3.3) g/day; (p=0.26) and women 7.6(3) vs 7.9(3.2) g/day; (p=0.43) were also similar for the two samples, as was salt excretion across age groups (p=0.72). The cost of obtaining each 24 h urine sample was two times greater for the random compared to volunteer samples ($A62 vs $A31). CONCLUSIONS: The estimated salt consumption derived from the two samples was comparable and was not substantively different to estimates obtained from other surveys. In countries where salt is pervasive and cannot easily be avoided, estimates of consumption obtained from volunteer samples may be valid and less costly.