Creactive protein Microbiologic Studies

• Culture and susceptibility testing

FIGURE 69-2. Important bacterial pathogens classified according to Gram stain and morphologic characteristics. (From Rybak MJ, Aeschlimann JR. Laboratory tests to direct antimicrobial pharma-

cotherapy. In: In DiPiro JT, Talbert RL, Yee GC, et al., eds. Pharmacotherapy: A Pathophysiologic Approach. 6th ed. New York: McGraw-Hill; 2005:1894.)

cotherapy. In: In DiPiro JT, Talbert RL, Yee GC, et al., eds. Pharmacotherapy: A Pathophysiologic Approach. 6th ed. New York: McGraw-Hill; 2005:1894.)

FIGURE 69-3. Macrotube minimal inhibitory concentration (MIC) determination. The growth control (C), 0.5 mg/dL, and 1 mg/dL tubes are visibly turgid, indicating bacterial growth. The MIC is read as the first clear test tube (2 mg/dL). (From Rybak MJ, Aeschlimann JR. Laboratory tests to direct antimicrobial pharmacotherapy. In: In DiPiro JT, Talbert RL, Yee GC, et al., eds. Pharmacotherapy: A Pathophysiologic Approach. 6th ed. New York: McGraw-Hill; 2005:1897.)

FIGURE 69-3. Macrotube minimal inhibitory concentration (MIC) determination. The growth control (C), 0.5 mg/dL, and 1 mg/dL tubes are visibly turgid, indicating bacterial growth. The MIC is read as the first clear test tube (2 mg/dL). (From Rybak MJ, Aeschlimann JR. Laboratory tests to direct antimicrobial pharmacotherapy. In: In DiPiro JT, Talbert RL, Yee GC, et al., eds. Pharmacotherapy: A Pathophysiologic Approach. 6th ed. New York: McGraw-Hill; 2005:1897.)

Culture and susceptibility testing provides additional information to the clinician to guide appropriate therapy. Specimens are placed in or on culture media that provide the proper growth conditions. Once the bacteria grow on culture media, they can be identified through a variety of biochemical tests. Once a pathogen is identified, susceptibility tests can be performed to various antimicrobial agents. The minimum inhibitory concentration (MIC) is a standard susceptibility test. The MIC is the lowest concentration of antimicrobial that inhibits visible bacterial growth after approximately 24 hours (Fig. 69-3). Breakpoint and MIC values determine if the organism is susceptible (S), intermediate (I), or resistant (R) to an antimicrobial. The breakpoint is the concentration of the antimicrobial that can be achieved in the serum after a normal or standard dose of that antimicrobial. If the MIC is below the breakpoint, the organism is considered to be susceptible to that agent. If the MIC is above the breakpoint, the organism is said to be resistant. Reported culture and susceptibility results may not provide MIC values but report the S, I, and R results.

O In general, bacterial cultures should be obtained prior to initiating antimicrobial therapy in patients with a systemic inflammatory response, risk factors for antimicro-

bial resistance, or infections where diagnosis or antimicrobial susceptibility is uncertain. The decision to culture depends on the sensitivity and specificity of the physical findings, diagnostic examination findings, and whether or not the pathogens are readily predictable. Culture and susceptibility testing usually is not warranted in a young, otherwise healthy woman who presents with signs and symptoms consistent with a urinary tract infection (UTI) because the primary pathogen, Escherichia coli, is readily predictable. Cultures and susceptibility testing are routine for sterile-site specimens (e.g., blood and spinal fluid), as well as for material presumed to be infected (e.g., material obtained from joints and abscesses). Cultures need to be interpreted with caution. Poor specimen collection technique and processing speed can result in misleading information and inappropriate use of antimicrobials.

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