Chapter 17: Microbiology and Infectious Disease

I. Microbiology

1. Collection of Specimens for Blood Culture
   1. Preparation: Proper specimen collection is essential to minimize contamination. Clean venipuncture site with 70% isopropyl ethyl alcohol. Apply tincture of iodine or 10% povidone-iodine and allow to dry for at least 1 min, or scrub site with 2% chlorhexidine for 30 seconds and allow to dry for 30 seconds for dry sites, scrub for 2 minutes and allow to dry for 1 minute for moist sites. Clean blood culture bottle injection site with alcohol only
   2. Collection: Ideally, two sets of cultures of equal blood volume should be obtained for each febrile episode, based on patient weight: <8 kg, 1–3 mL each; 8–13 kg, 4–5 mL; 14–25 kg, 10–15 mL each; >25 kg, 20–30 mL each (adapted from Kaditis et al., 1996¹). Total blood volume drawn should not exceed more than 1% of the patient's total blood volume
2. Rapid Microbiologic Identification of Common Aerobic Bacteria (Fig. 17-1)
3. Choosing Appropriate Antibiotic Based on Sensitivities
   1. Definitions^2,3:
      1. Minimum inhibitory concentration (MIC): Lowest concentration of an antimicrobial agent that prevents visible growth after an 18- to 24-hour incubation period
      2. Minimum bactericidal concentration (MBC): Lowest concentration of an antimicrobial agent that kills the organism, as measured by subculturing to antibiotic-free media after 18- to 24-hour incubation
   2. Mistakes to avoid when selecting antibiotics based on sensitivity profiles (Table 17-1): Clinically significant, common discrepancies between in vitro (laboratory reported) and in vivo antibiotic sensitivity profiles that may result in inappropriate antibiotic treatment


FIGURE 17-1 Algorithm demonstrating identification of aerobic bacteria. Numbers in parentheses indicate the time required for the tests.