Introduction. Antibody testing for evidence of a recent Bordetella pertussis infection by estimating anti-pertussis toxin immunoglobulin G (anti-PT-IgG) titres by enzyme-linked immunosorbent assays is often recommended for those with a cough lasting more than 14 days. Interpreting results varies, with studies recommending different anti-PT-IgG titre thresholds for assigning positivity. In England, early work looking at antibody titre distributions for samples submitted from April 2010 to July 2012 found an optimal threshold of greater than 70 IU ml−1 for good sensitivity, specificity and positive predictive value. Aim. The aim of this study is to use the same mixture modelling technique to determine if the 70 IU ml−1 threshold remains appropriate when assessing data before, during and after the outbreak of pertussis in 2011–2012. Methods. We reviewed titres for all serology-tested samples in England between 1 July 2008 to 30 June 2022. IgG titres were used to calculate the positivity based on the current threshold of 70 IU ml−1, the median duration of cough for individuals who tested positive and, through mixture modelling, the sensitivity, specificity, positive and negative predictive values (PPV and NPV) of assay thresholds. Results. Positivity rates increased from 21.7 % prior to the outbreak to 30.3 % during the outbreak and dropped to 25.1 % post-outbreak; similar to estimates from the mixture model of 20.5, 33.3 and 28.7 %, respectively. Although the estimated sensitivity dropped during and after the outbreak when applying the 70 IU ml−1 threshold, the PPV remained high and therefore no change to this threshold is warranted. Conclusion. Mixture modelling is a useful tool to establish thresholds, but reassessment should also be done when there have been changes to prevalence and/or testing regimes to determine whether there have been any changes in sensitivity, specificity, PPV, and NPV and whether the threshold should be revised.

中文翻译：

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百日咳博德特氏菌血清学样本的混合模型，用于评估抗百日咳毒素免疫球蛋白 G 阳性滴度阈值：英格兰 2008-2022

介绍。对于咳嗽持续超过 14 天的患者，通常建议通过酶联免疫吸附测定法估计抗百日咳毒素免疫球蛋白 G（抗 PT-IgG）滴度，进行抗体检测，以发现近期感染百日咳博德特氏菌的证据。对结果的解释各不相同，研究建议使用不同的抗 PT-IgG 滴度阈值来分配阳性。在英格兰，对2010年4月至2012年7月提交的样本的抗体滴度分布进行了早期研究，发现大于70 IU ml -1的最佳阈值可以获得良好的敏感性、特异性和阳性预测值。目的。本研究的目的是使用相同的混合建模技术来确定在评估 2011-2012 年百日咳爆发之前、期间和之后的数据时70 IU ml -1阈值是否仍然合适。方法。我们审查了2008年7月1日至2022年6月30日期间英格兰所有血清学检测样本的滴度。IgG滴度用于根据当前阈值70 IU ml -1计算阳性，即检测呈阳性的个体咳嗽的中位持续时间并通过混合建模确定检测阈值的灵敏度、特异性、阳性和阴性预测值（PPV 和 NPV）。结果。阳性率从疫情前的21.7%上升至疫情期间的30.3%，并下降至疫情后的25.1%；与混合模型的估计值相似，分别为 20.5%、33.3% 和 28.7%。尽管在应用 70 IU ml -1阈值时，估计的敏感性在爆发期间和爆发后下降，但 PPV 仍然很高，因此无需更改该阈值。结论。混合模型是建立阈值的有用工具，但当患病率和/或测试制度发生变化时，也应进行重新评估，以确定敏感性、特异性、PPV 和 NPV 是否有任何变化，以及阈值是否应该予以修订。