Sørensen L L, Wachmann H, Alban L
Danish Meat Association, Axelborg, 3 Axeltorv, DK-1609 Copenhagen V, Denmark.
Vet Microbiol. 2007 Jan 31;119(2-4):213-20. doi: 10.1016/j.vetmic.2006.08.025. Epub 2006 Sep 20.
Pooling of samples might be an effective means to increase cost-effectiveness in routine surveillance. The present study assessed the effect on the sensitivity of detection of Salmonella when pooling swab samples from swine carcasses compared to individual analyses. A total of 18,984 samples from nine Danish swine abattoirs were collected during 1 year, covering 2017 slaughter days. At each abattoir, swab samples were taken on a daily basis from 10 carcasses randomly selected. From each carcass, an area of 3 cm x 100 cm was swabbed. Five of these samples were analysed individually and the other five were analysed as one pooled sample. Standard culture methods were used. A logistic regression model was built, where the response was whether a sample was Salmonella positive or not. The explanatory factors were abattoir, type of sampling (individual or pooled sample), and season of year 2000 (four quarters). The odds ratio (OR) of the effect of type of sampling in the logistic model accounting for abattoir and season was interpreted as the conversion factor between pooled and individual sample prevalence. The results of the individually analysed samples showed a low prevalence of Salmonella (1.4%). When Salmonella was isolated, mostly only one positive sample was found among the five individually analysed samples per slaughter day. On a few days >1 positive samples' were found (9 out of 2017 days approximately 0.4%). The pooled sample prevalence was 4.1%. Because the individual prevalence was low, the pooled sample prevalence would have been around five times higher than the individual-level prevalence-if there had been no loss of sensitivity. However, we found that due to loss of sensitivity the pooled prevalence was only three times higher (OR = 2.7; CI 2.0-3.7). Therefore, a conversion factor of 3 instead of 5 should be applied to calculate the individual prevalence from a pooled prevalence. This approach has been used in the national surveillance of Danish pork since 2001. The estimated conversion factor and accept of pooling samples do not necessarily apply to a population with a higher prevalence or to other types of samples (e.g. faeces or lymph nodes) or diagnostic procedures.
样本合并可能是提高常规监测成本效益的有效手段。本研究评估了与单独分析相比，合并来自猪胴体的拭子样本对沙门氏菌检测灵敏度的影响。在1年期间，从丹麦9个生猪屠宰场共收集了18984个样本，涵盖2017个屠宰日。在每个屠宰场，每天从随机选择的10头猪胴体上采集拭子样本。从每头猪胴体上擦拭3厘米×100厘米的面积。其中5个样本单独分析，另外5个样本作为一个合并样本分析。采用标准培养方法。建立了一个逻辑回归模型，其中响应变量是样本是否为沙门氏菌阳性。解释变量是屠宰场、采样类型（单独样本或合并样本）以及2000年的季节（四个季度）。在考虑屠宰场和季节的逻辑模型中，采样类型的效应的优势比（OR）被解释为合并样本患病率与单独样本患病率之间的转换因子。单独分析样本的结果显示沙门氏菌患病率较低（1.4%）。当分离出沙门氏菌时，每个屠宰日的5个单独分析样本中大多仅发现1个阳性样本。在少数几天发现了>1个阳性样本（2017天中有9天，约0.4%）。合并样本患病率为4.1%。由于单独患病率较低，如果没有灵敏度损失，合并样本患病率将比单独水平的患病率高约五倍。然而，我们发现由于灵敏度损失，合并患病率仅高两倍（OR = 2.7；CI 2.0 - 3.7）。因此，应应用3而不是5的转换因子从合并患病率计算单独患病率。自2001年以来，这种方法已用于丹麦猪肉的国家监测。估计的转换因子和样本合并的可接受性不一定适用于患病率较高的人群或其他类型的样本（如粪便或淋巴结）或诊断程序。
