How to Choose an Antibiotic?


Dr Bakul Parekh, Mumbai    19 January 2018

Proper collection of cultures - Collect specimen before starting antibiotics; collect sample with due precautions; send sample from correct site; tissue, fluid, aspirate are better than swabs; expedite transport; send adequate amount of sample.

  1. Follow standard guideline recommendations for choosing antibiotic.
  2. Tips for blood culture - As the quantity of blood sent for culture is increased, the chances of a positive culture improve.
  1. In children, send at least 4-8 mL of blood (in one or two aliquots - paired samples or set).
  2. In neonates, smaller amounts of blood may still yield positive result; send correct ratio of blood to culture medium (1:5).

3. If there is delay in transport to lab, refrigerate samples from contaminated sites (urine, stool). Do not refrigerate samples from sterile sites (CSF, blood cultures, body fluids).

4. Despite in vitro susceptibility, ciprofl oxacin and ceftazidime are not preferred agents for S. aureus. Clindamycin sensitivity needs to be confi rmed by D test. Erythromycin is bacteriostatic, so not preferred. Vancomycin and teicoplanin should be reserved for methicillin-resistant S. aureus.

5. DST reports should be available as ‘S’, ‘I’, ‘R’. Clinical judgment is required in choosing antibiotics instead of blindly following DST reports as in vivo susceptibility may not be the same as in vitro.

6. Non-culture methods for bacteriological identifi cation:

  1. Microscopy of organism
  2. Antigen testing
  3. Antibody detection
  4. PCR.

7. Principles of choosing an antibiotic - Select drug with the narrowest spectrum effective for the pathogen: to reduce damage to resident fl ora, to save higher/ broad-spectrum for future care and to reduce the danger of inducing drug resistance in the patient and the environmental fl ora; select drug that is likely to be less toxic; use single agent as far as possible; in vitro and in vivo susceptibility may differ.

8. An ideal antibiotic is the one that harms the bacteria, not the host; has a narrow spectrum; has minimal adverse effects; can be administered through various routes of administration - IV, IM, oral.

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