暴露测量错分对研究真实性的影响

通过数据模拟的方式，假定某人群的总人口为2 000万，人群疾病感染率为0.1%~5.0%，模拟不同灵敏度（99.0%、99.5%、100.0%）和特异度（97.0%、97.5%、98.0%、98.5%、99.0%、99.5%、99.9%）的组合情况，计算各组合的PPV、真阳性数和假阳性数。

在低感染率（≤5.0%）状态下，与灵敏度相比，特异度对PPV的影响更大，且随着感染率的下降，特异度提高时PPV的增幅加大。当感染率>1.0%时，特异度越趋近99.9%，PPV越接近100.0%；而当感染率<1.0%时，PPV的最大值也仅约为90.0%。当人群感染率为0.1%、灵敏度≥99.0%时，在2 000万的人群中筛检试验可以发现的真阳性人数约为2万人；当特异度为97.0%时，假阳性人数约为 59.9万人，PPV为3.2%；当特异度达到99.9%时，假阳性人数降到约2万人，PPV升高到50.0%。当人群感染率为1.0%时，灵敏度≥99.0%和特异度≥97.0%的筛检试验在2 000万的人群中可发现的真阳性数为19.8万~20.0万人；当特异度由97.0%升高至99.9%时，假阳性人数由59.4万人降至2万人。当人群感染率为5.0%时，灵敏度≥99.0%和特异度≥97.0%的筛检试验在2 000万的人群中可发现的真阳性数为99.0万~100.0万人；当特异度由97.0%升高至99.9%时，假阳性人数由57.0万人降至1.9万人。在总人口为 2 000万的人群中，当感染率≤5.0%时，即使灵敏度和特异度分别达到最大值100.0%和99.9%，仍存在约2万例的假阳性。

当人口数量较多而感染率较低时，运用筛检试验进行大规模人群筛查除了要提高特异度外，大量假阳性所产生的问题不可忽视。

Medical screening

To explore the positive predictive value （PPV） and false positive （FP） number of screening test in mass testing when the prevalence of infection is low.Assuming a population of 20 million with the prevalence of disease infection ranging from 0.1% to 5.0%， PPV， true positive （TP） and FP numbers were calculated under different scenarios of combination of sensitivity （99.0%， 99.5%， and 100.0%） with specificity （97.0%， 97.5%， 98.0%， 98.5%， 99.0%， 99.5%， and 99.9%）.For low infection prevalence （≤5.0%）， specificity has a greater impact on PPV than sensitivity； with the decrease of infection prevalence， the increase in PPV elevates when the specificity increases. When the infection prevalence is >1.0%， the closer the specificity is to 99.9%， the closer the PPV is to 100.0%. However， when the infection prevalence is <1.0%， the maximum PPV is only about 90.0%. When the infection rate is 0.1%， a screening test with more than 99.0% sensitivity could detect about 20 thousand TP cases in a population of 20 million. Additionally， the FP and PPV are estimated to be 599 thousand and 3.2% if the specificity is 97.0%， and 20 thousand and 50.0% if the specificity is 99.9%. When the infection rate is 1.0%， a screening test with ≥99.0% sensitivity and ≥97.0% specificity could detect about 0.198‒0.200 million TP cases； and the number of FP decreases from 594 thousand to 20 thousand when the specificity increases from 97.0% to 99.9%. When the infection rate is 5.0%， a screening test with ≥99.0% sensitivity and ≥97.0% specificity could detect about 0.99‒1.00 million TP cases； and the number of FP decreases from 570 thousand to 19 thousand when the specificity increases from 97.0% to 99.9%. When the infection prevalence is ≤5.0% in a total population of 20 million， there are about 20，000 FP cases even if the sensitivity and specificity reach the maximum values of 100.0% and 99.9%， respectively.When the population is large and the infection prevalence is low， in addition to improving the specificity of the screening test in mass testing， the problem of a large number of false positives cannot be ignored.