The cefazolin inoculum effect (CzIE) has been associated with therapeutic failures and mortality in invasive methicillin-susceptible Staphylococcus aureus (MSSA) infections. A diagnostic test to detect the CzIE is not currently available. We developed a rapid (∼3 h) CzIE colorimetric test to detect staphylococcal-β-lactamase (BlaZ) activity in supernatants after ampicillin induction. The test was validated using 689 bloodstream MSSA isolates recovered from Latin America and the United States. The cefazolin MIC determination at a high inoculum (10⁷ CFU/ml) was used as a reference standard (cutoff ≥16 μg/ml). All isolates underwent genome sequencing. A total of 257 (37.3%) of MSSA isolates exhibited the CzIE by the reference standard method. The overall sensitivity and specificity of the colorimetric test was 82.5% and 88.9%, respectively. Sensitivity in MSSA isolates harboring type A BlaZ (the most efficient enzyme against cefazolin) was 92.7% with a specificity of 87.8%. The performance of the test was lower against type B and C enzymes (sensitivities of 53.3% and 72.3%, respectively). When the reference value was set to ≥32 μg/ml, the sensitivity for isolates carrying type A enzymes was 98.2%. Specificity was 100% for MSSA lacking blaZ. The overall negative predictive value ranged from 81.4% to 95.6% in Latin American countries using published prevalence rates of the CzIE. MSSA isolates from the United States were genetically diverse, with no distinguishing genomic differences from Latin American MSSA, distributed among 18 sequence types. A novel test can readily identify most MSSA isolates exhibiting the CzIE, particularly those carrying type A BlaZ. In contrast to the MIC determination using high inoculum, the rapid test is inexpensive, feasible, and easy to perform. After minor validation steps, it could be incorporated into the routine clinical laboratory workflow.

中文翻译：

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甲氧西林敏感金黄色葡萄球菌头孢唑啉接种效果的快速检测试验

头孢唑啉接种物效应 (CzIE) 与侵袭性甲氧西林敏感金黄色葡萄球菌(MSSA) 感染的治疗失败和死亡率有关。目前没有检测 CzIE 的诊断测试。我们开发了一种快速（~3 小时）CzIE 比色测试，以检测氨苄青霉素诱导后上清液中的葡萄球菌-β-内酰胺酶（BlaZ）活性。该测试使用从拉丁美洲和美国回收的 689 种血流 MSSA 分离株进行了验证。在高接种量（10 ⁷CFU/ml) 用作参考标准（临界值≥16 μg/ml）。所有分离株都进行了基因组测序。通过参考标准方法，共有 257 个 (37.3%) MSSA 分离株表现出 CzIE。比色测试的总体敏感性和特异性分别为 82.5% 和 88.9%。携带 A 型 BlaZ（对头孢唑啉最有效的酶）的 MSSA 分离株的敏感性为 92.7%，特异性为 87.8%。该测试对 B 型和 C 型酶的性能较低（灵敏度分别为 53.3% 和 72.3%）。当参考值≥32 μg/ml时，对携带A型酶的分离株的敏感性为98.2%。缺乏blaZ 的MSSA 的特异性为 100%. 使用已公布的 CzIE 患病率，拉丁美洲国家的总体阴性预测值为 81.4% 至 95.6%。来自美国的 MSSA 分离株具有遗传多样性，与拉丁美洲 MSSA 没有明显的基因组差异，分布在 18 种序列类型中。一种新的测试可以很容易地识别出大多数表现出 CzIE 的 MSSA 分离株，尤其是那些携带 A 型 BlaZ 的分离株。与使用高接种量的 MIC 测定相比，快速测试便宜、可行且易于执行。经过小的验证步骤后，它可以被纳入常规临床实验室工作流程。