Reliable quantification of the potential for equations based on spot urine samples to estimate population salt intake: protocol for a systematic review and meta-analysis

Background: Methods based on spot urine samples (a single sample at one time-point) have been identified as a possible
alternative approach to 24-hour urine samples for determining mean population salt intake.
Objective: The aim of this study is to identify a reliable method for estimating mean population salt intake from spot urine
samples. This will be done by comparing the performance of existing equations against one other and against estimates derived
from 24-hour urine samples. The effects of factors such as ethnicity, sex, age, body mass index, antihypertensive drug use, health
status, and timing of spot urine collection will be explored. The capacity of spot urine samples to measure change in salt intake
over time will also be determined. Finally, we aim to develop a novel equation (or equations) that performs better than existing
equations to estimate mean population salt intake.
Methods: A systematic review and meta-analysis of individual participant data will be conducted. A search has been conducted
to identify human studies that report salt (or sodium) excretion based upon 24-hour urine samples and spot urine samples. There
were no restrictions on language, study sample size, or characteristics of the study population. MEDLINE via OvidSP (1946-present),
Premedline via OvidSP, EMBASE, Global Health via OvidSP (1910-present), and the Cochrane Library were searched, and two
reviewers identified eligible studies. The authors of these studies will be invited to contribute data according to a standard format.
Individual participant records will be compiled and a series of analyses will be completed to: (1) compare existing equations for
estimating 24-hour salt intake from spot urine samples with 24-hour urine samples, and assess the degree of bias according to
key demographic and clinical characteristics; (2) assess the reliability of using spot urine samples to measure population changes
in salt intake overtime; and (3) develop a novel equation that performs better than existing equations to estimate mean population
salt intake.
Results: The search strategy identified 538 records; 100 records were obtained for review in full text and 73 have been confirmed
as eligible. In addition, 68 abstracts were identified, some of which may contain data eligible for inclusion. Individual participant
data will be requested from the authors of eligible studies.
Conclusions: Many equations for estimating salt intake from spot urine samples have been developed and validated, although
most have been studied in very specific settings. This meta-analysis of individual participant data will enable a much broader
understanding of the capacity for spot urine samples to estimate population salt intake.