Sard Brian, Bailey Mary Christine, Vinci Robert
Department of Pediatric Emergency Medicine, Division of Pediatric Emergency Medicine, Boston University School of Medicine and Boston Medical Center, Boston, MA 02118, USA.
Pediatr Emerg Care. 2006 May;22(5):295-300. doi: 10.1097/01.pec.0000215137.51909.16.
Blood cultures are commonly included in the evaluation of febrile children younger than 3 years without focal source of infection. Clinicians treat patients with a positive blood culture before final identification of the organism. Their treatment might include reevaluation in the emergency department (ED), additional tests, parenteral antibiotics, and hospital admission even for children who ultimately have false-positive (FP) blood cultures. The advent of pneumococcal conjugate vaccine (PCV) has made occult bacteremia less common, decreasing the likelihood that a positive blood culture result before final organism identification will be a true pathogen. This study will identify the characteristics of patients with FP blood cultures in the post-PCV era.
Charts were reviewed of all children ages 1 to 36 months with a temperature of at least 38.08 degrees C who had a blood culture obtained in our community hospital ED from January 1997 to January 2005.
Bacteria grew in 106 (3.5%) out of 2971 blood cultures. True positives (TPs), defined as true pathogens, had a prevalence of 0.7%, representing 19.8% of positives. FPs, defined as contaminants, occurred in 2.8% of cultures, representing 80.2% of positives. Patients with FP cultures had lower mean white blood cell (WBC) counts (10.51 x 10(9)/L vs. 16.95 x 10(9)/L; P = 0.0001) and lower mean presenting temperatures (38.8 degrees C vs. 39.4 degrees C; P = 0.005). FPs had longer time to positivity (34.6 vs. 17.7 hours; P = 0.001) than TPs. A culture with a Gram stain suggestive of a contaminant, time to positivity greater than 24 hours and a WBC of less than 15 x 10(9)/L had a PPV for an FP of 97%. When analysis was restricted to well-appearing children age 2 to 36 months with temperature of more than 39 degrees C without focal source of infection who were discharged from the ED, these three criteria had a PPV for an FP of 100%. In these highly febrile children, the FPs had significantly lower WBCs (9.14 x 10(9)/L vs. 22.84 x 10(9)/L; P = 0.0001) and longer time topositivity (33.4 vs. 19.8 hours; P = 0.007) than TPs. The likelihood of obtaining FP cultures increased after the introduction of PCV from 62.5% to 87.8% odds ratio, 4.3; 95%confidence internal, 1.44-13.38).
In the post-PCV era, the majority of blood culture results will be FPs. FP cultures are predictable in febrile children with WBC counts less than 15.00 x 10(9)/L, time to positivity of more than 24 hours, and a Gram stain result suggestive of a contaminant. Prospective studies applying these criteria to the at-risk population for occult bacteremia are indicated.
血培养通常用于评估3岁以下无局灶性感染源的发热儿童。临床医生在最终鉴定出病原体之前就会对血培养阳性的患者进行治疗。他们的治疗可能包括在急诊科（ED）进行重新评估、额外的检查、胃肠外抗生素治疗以及住院治疗，即使对于最终血培养结果为假阳性（FP）的儿童也是如此。肺炎球菌结合疫苗（PCV）的出现使隐匿性菌血症变得不那么常见，降低了在最终鉴定出病原体之前血培养阳性结果为真正病原体的可能性。本研究将确定PCV时代血培养假阳性患者的特征。
回顾了1997年1月至2005年1月在我们社区医院急诊科进行血培养的所有1至36个月、体温至少为38.08摄氏度的儿童的病历。
2971份血培养中有106份（3.5%）培养出细菌。定义为真正病原体的真阳性（TP）患病率为0.7%，占阳性结果的19.8%。定义为污染物的FP在2.8%的培养物中出现，占阳性结果的80.2%。血培养为FP的患者平均白细胞（WBC）计数较低（10.51×10⁹/L对16.95×10⁹/L；P = 0.0001），平均就诊时体温较低（38.8摄氏度对39.4摄氏度；P = 0.005）。与TP相比，FP达到阳性的时间更长（34.6小时对17.7小时；P = 0.001）。革兰氏染色提示为污染物、达到阳性的时间超过24小时且WBC低于15×10⁹/L的培养物，其FP的阳性预测值（PPV）为97%。当分析仅限于从ED出院的2至36个月、体温超过39摄氏度、无局灶性感染源且外观良好的儿童时，这三个标准的FP的PPV为100%。在这些高热儿童中，与TP相比，FP的WBC显著更低（9.14×10⁹/L对22.84×10⁹/L；P = 0.0001），达到阳性的时间更长（33.4小时对19.8小时；P = 0.007）。引入PCV后获得FP培养物的可能性增加（优势比为4.3；95%置信区间为1.44 - 13.38，从62.5%增至87.8%）。
在PCV时代，大多数血培养结果将为FP。对于WBC计数低于15.00×10⁹/L、达到阳性的时间超过24小时且革兰氏染色结果提示为污染物的发热儿童，FP培养是可预测的。建议对隐匿性菌血症的高危人群应用这些标准进行前瞻性研究。
