Zenilman J M, Miller W C, Gaydos C, Rogers S M, Turner C F
Infectious Diseases Division Johns Hopkins University School of Medicine, Baltimore, MD, USA.
Sex Transm Infect. 2003 Apr;79(2):94-7. doi: 10.1136/sti.79.2.94.
Nucleic acid amplification tests have facilitated field based STD studies and increased screening activities. However, even with highly specific tests, the positive predictive value (PPV) of such tests may be lower than desirable in low prevalence populations. We estimated PPVs for a single LCR test in a population survey in which positive specimens were retested.
The Baltimore STD and Behavior Survey (BSBS) was a population based behavioural survey of adults which included collecting urine specimens to assess the prevalence of gonorrhoea and chlamydial infection. Gonorrhoea and chlamydial infection were diagnosed by ligase chain reaction (LCR). Nearly all positive results were retested by LCR. Because of cost considerations, negative results were not confirmed. Predicted curves for the PPV were calculated for a single testing assuming an LCR test sensitivity of 95%, and test specificities in the range 95.0%-99.9%, for disease prevalences between 1% and 10%. Positive specimens were retested to derive empirical estimates of the PPV of a positive result on a single LCR test.
579 participants age 18-35 provided urine specimens. 20 (3.5%) subjects initially tested positive for chlamydial infection, and 39 (6.7%) tested positive for gonococcal infection. If positive results on the repeat LCR are taken as confirmation of a "true" infection, the observed PPV for the first LCR testing was 89.5% for chlamydial infection and 83.3% for gonorrhoea. This is within the range of theoretical PPVs calculated from the assumed sensitivities and specificities of the LCR assays.
Empirical performance of a single LCR testing approximated the theoretically predicted PPV in this field study. This result demonstrates the need to take account of the lower PPVs obtained when such tests are used in field studies or clinical screening of low prevalence populations. Repeat testing of specimens, preferably with a different assay (for example, polymerase chain reaction), and disclosure of the non-trivial potential for false positive test results would seem appropriate in all such studies.
核酸扩增检测推动了基于现场的性传播疾病研究并增加了筛查活动。然而，即便采用高度特异的检测方法，此类检测在低流行率人群中的阳性预测值（PPV）可能仍低于预期。我们在一项对阳性标本进行复测的人群调查中估算了单次连接酶链反应（LCR）检测的PPV。
巴尔的摩性传播疾病与行为调查（BSBS）是一项针对成年人的基于人群的行为调查，其中包括收集尿液标本以评估淋病和衣原体感染的流行率。淋病和衣原体感染通过连接酶链反应（LCR）诊断。几乎所有阳性结果都通过LCR复测。出于成本考虑，未对阴性结果进行确认。针对单次检测计算了PPV的预测曲线，假设LCR检测灵敏度为95%，检测特异性在95.0% - 99.9%范围内，疾病流行率在1%至10%之间。对阳性标本进行复测以得出单次LCR检测阳性结果的PPV的经验估计值。
579名年龄在18 - 35岁的参与者提供了尿液标本。20名（3.5%）受试者最初衣原体感染检测呈阳性，39名（6.7%）受试者淋病检测呈阳性。如果将复测LCR的阳性结果视为“真正”感染的确认，那么首次LCR检测中观察到的衣原体感染PPV为89.5%，淋病为83.3%。这在根据LCR检测假定的灵敏度和特异性计算出的理论PPV范围内。
在这项现场研究中，单次LCR检测的实际表现接近理论预测的PPV。该结果表明，在低流行率人群的现场研究或临床筛查中使用此类检测时，需要考虑到所获得的较低PPV。在所有此类研究中，对标本进行复测（最好采用不同的检测方法，如聚合酶链反应）并告知假阳性检测结果存在显著可能性似乎是合适的。
