CID：经验性治疗可致艰难梭菌诊断测试出现假阴性

艰难梭菌感染（CDI）疑似患者常常在收集用于诊断测试的粪便样本前，就先接受经验性治疗。目前尚不清楚这种经验性治疗是否会导致CDI诊断测试结果出现假阴性。因此，来自美国凯斯西储大学医学院的医学博士Curtis J. Donskey 等人展开一项研究，研究结果在线发表于2013年5月3日的《临床感染性疾病》（Clinical Infectious Diseases）杂志上。作者发现，当粪便样本的收集存在延误时，对CDI疑似病例采取经验性治疗可能会导致诊断测试结果出现假阴性。
研究人员开展了一项针对CDI患者的前瞻性试验，以判断CDI治疗过程中测试结果何时由阳性转变为阴性，包括毒素B基因聚合酶链式反应（PCR）、谷氨酸脱氢酶检测以及产毒培养。他们评估了CDI测试持续阳性的频率和危险因素。对于接受经验性治疗的患者，评估其CDI检测结果由阳性转变为阴性的转变率，检测时间点分别是顺序检验（test order）时以及至临床人员收集测试用粪便样本时。
研究结果如下，51例CDI患者的PCR、谷氨酸脱氢酶检测和产毒培养结果的转变率相当。在接受1天、2天及3天治疗之后，PCR测试结果分别有14%、35%和45%由阳性转为阴性。年龄增长以及感染脉冲场凝胶电泳分型为NAP1的菌株这二者与PCR结果持续阳性相关。对于顺序检验时被诊断为CDI的患者，9例接受规定CDI经验性治疗的患者中有4例（44%）的临床粪便样本PCR结果转变为阴性，相比之下，23例未接受经验性治疗的患者无一发生转变（0%，P=0.004）。
研究发现，如果粪便样本的收集存在延迟，那么对CDI疑似病例采取经验性治疗可能会导致PCR结果出现假阴性。这一研究存在一定的局限性。首先，本研究在一所以男性患者为主的医疗中心开展。其次，未将门诊患者纳入研究。再者，样本量太小以致无法进行进一步分析。未来需要更大规模的研究来确证这一结果。
与艰难梭菌相关的拓展阅读：

• AJG：粪菌移植有效治疗复发性艰难梭菌感染
• Am J Infect Control：入院时靶向筛查艰难梭菌或可识别出多数定殖患者
• AJG：肠道艰难梭菌诊治指南
• Clin Infect Dis:儿童艰难梭菌感染率明显上升
• NAT GENET :艰难梭菌全球流行原因被识别
• NEJM：非达霉素治疗艰难梭菌感染不逊于万古霉素 4周时更胜一筹
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Does empirical Clostridium difficile Infection (CDI) therapy result in false-negative CDI diagnostic test results?
Background
Patients with suspected Clostridium difficile infection (CDI) often receive empirical therapy prior to collection of stool specimens for diagnostic testing. The likelihood that such empirical therapy might result in false-negative CDI test results is unknown.
Methods
We conducted a prospective study of CDI patients to determine the time to conversion of CDI test results, including polymerase chain reaction (PCR) for toxin B genes, glutamate dehydrogenase, and toxigenic culture, from positive to negative during CDI therapy. We evaluated the frequency of and risk factors for persistence of positive CDI tests. For patients receiving empirical therapy, we assessed the frequency of conversion of positive CDI test results at the time of the test order to negative by the time clinical staff collected stool specimens for testing.
Results
For 51 CDI patients, PCR, glutamate dehydrogenase, and toxigenic culture results converted to negative at similar rates. For PCR, 14%, 35%, and 45% of positive CDI tests converted to negative after 1, 2, and 3 days of treatment, respectively. Increased age and infection with North American pulsed-field gel electrophoresis strains were associated with persistent positive PCR results. For CDI patients diagnosed at the time of the test order, conversion to negative PCR results by the time clinical stool specimens were collected occurred in 4 of 9 (44%) patients who were prescribed empirical CDI therapy versus 0 of 23 (0%) who were not (P=0.004).
Conclusions
Empirical treatment for suspected CDI cases may result in false-negative PCR results if there are delays in stool specimen collection.