Human Nutrition Laboratory, Institute of Food Nutrition and Health, ETH Zurich, Zurich, Switzerland.
Seminar for Statistics, ETH Zurich, Zurich, Switzerland.
Am J Clin Nutr. 2023 Jun;117(6):1270-1277. doi: 10.1016/j.ajcnut.2023.03.012. Epub 2023 May 4.
The habitual/usual iodine intake and the prevalence of iodine inadequacy may be estimated from spot urinary iodine concentrations in cross-sectional studies by collecting a repeat spot urine in a subgroup of the study population and accounting for within-person variability in iodine intake. However, guidance on the required overall sample size (N) and the replicate rate (n) is lacking.
To determine the sample size (N) and replicate rate (n) needed to estimate the prevalence of iodine inadequacy in cross-sectional studies.
We used data from local observational studies conducted in women 17-49 y old in Switzerland (N = 308), South Africa (N = 154), and Tanzania (N = 190). All participants collected 2 spot urine samples. We calculated the iodine intake using urinary iodine concentrations and accounted for urine volume using urinary creatinine concentration. For each study population, we estimated the habitual iodine intake distribution and determined the prevalence of iodine intake below the average requirement using the Statistical Program to Assess habitual Dietary Exposure (SPADE). We used the obtained model parameters in power analyzes and estimated the prevalence of iodine inadequacy for different sample sizes (N = 400, 600, and 900) and replicate rates (n = 50, 100, 200, 400, 600, and 900).
The estimated prevalence (95% CI) of inadequate iodine intake was 21% (15, 28%), 5.1% (1.3, 8.7%), and 8.2% (3.4, 13%) for Swiss, South African, and Tanzanian women, respectively. An N of 400 women, with a repeated measure (n) in 100 women, achieved a satisfactory precision of the prevalence estimate in all study populations. Increasing the replicate rate (n) improved the precision more effectively than increasing the N of the study.
The sample size for cross-sectional studies aiming to assess the prevalence of inadequate iodine intake depend on the expected prevalence, the overall variance in intake, and the study design. However, an N of 400 participants with a repeated measure of 25% may be used as guidance when planning observational studies applying simple random sampling. This trial was registered at clinicaltrials.gov as NCT03731312.
通过在研究人群的亚组中重复采集单次尿液样本来收集点尿样，可以从横断面研究中估计习惯性/通常的碘摄入量和碘摄入不足的流行率，并考虑到个体内碘摄入量的变异性。然而，关于所需的总样本量（N）和重复率（n）的指南是缺乏的。
确定用于估计横断面研究中碘摄入不足流行率的样本量（N）和重复率（n）。
我们使用了在瑞士（N=308）、南非（N=154）和坦桑尼亚（N=190）进行的女性 17-49 岁的局部观察性研究的数据。所有参与者均采集了 2 份尿样。我们使用尿碘浓度来计算碘摄入量，并使用尿肌酐浓度来计算尿量。对于每个研究人群，我们估计了习惯性碘摄入量分布，并使用统计程序评估习惯性膳食暴露（SPADE）来确定碘摄入量低于平均需求量的流行率。我们使用获得的模型参数进行功效分析，并为不同的样本量（N=400、600 和 900）和重复率（n=50、100、200、400、600 和 900）估计了碘摄入不足的流行率。
瑞士、南非和坦桑尼亚女性碘摄入不足的估计流行率（95%CI）分别为 21%（15，28%）、5.1%（1.3，8.7%）和 8.2%（3.4，13%）。N=400 名女性，重复测量（n）为 100 名女性，在所有研究人群中均能获得对流行率估计的满意精度。增加重复率（n）比增加研究的 N 更有效地提高了精度。
旨在评估碘摄入不足流行率的横断面研究的样本量取决于预期的流行率、摄入的总体变异性和研究设计。然而，当计划应用简单随机抽样的观察性研究时，400 名参与者的重复测量 25%可能被用作指导。该试验在 clinicaltrials.gov 上注册为 NCT03731312。
