What Tests Do I Need to Get Antibiotics?
Infections caused by bacteria are treated with antibiotics. There are two ways to use them:
- Rational choice of antibiotic(s) based on culture and sensitivity
- Empiric therapy, based on experience and a clinical educated guess in the absence of definitive information
While the culture and sensitivity is the mainstay of identifying and treating infections, the white blood count (WBC) with differential can list the types of WBCs that are elevated, possibly steering one to an empiric diagnosis. For example, eosinophils or basophils will be elevated in parasitic infections; the neutrophils are highly elevated in acute bacterial infections; and lymphocytes in viral illnesses or pertussis (whooping cough).
Culture and Sensitivity
Antibiotics can be chosen based on the exact bacteria causing the infection and the choice steered toward those for which the bacteria are not resistant.
- Culture: A sample from the infection, e.g., a throat swab of someone’s tonsils with tonsillitis, is retrieved and the sample deposited onto a culture medium to grow it.
- Sensitivity: Different antibiotics are applied to the culture and the results will demonstrate which ones kill the bacteria (bactericidal), which ones stop their growth or reproduction (bacteriostatic), and which ones don’t do anything at all (resistance).
Armed with this information, the physician can choose the best antibiotic based on the need for immediate action (e.g., sepsis), patient allergies, side effects (e.g., caution used in patient with kidney disease when using antibiotics cleared by or potentially toxic to the kidneys), expense, tolerability, oral vs intravenous/intramuscular, and length of therapy (some have a longer time for a full course, which can undermine patient compliance).
Empiric therapy is initiating an antibiotic based on a physician’s experience and knowledge of classical bacterial susceptibilities. It is used for two reasons:
- When the infection is so severe waiting the 2-3 days for the culture and sensitivity results could mean there is no patient alive to treat.
- When a physician is fairly certain that empiric therapy will give a 2-3 day head start that can be fine-tuned with a change in antibiotics should the culture and sensitivity so indicate.
Diagnosing an infection requires a sample of the infectious organism. While this may be simple with a ruptured skin abscess (a swab to the purulent exudate), Strep throat (a throat swab), or even sepsis (drawing blood), it may be more difficult with an infection in the lungs or abdomen. There are diagnostic procedures that can render such difficult retrievals:
- bronchoscopy (direct visualization of the lungs)
- “milking” of the prostate (rectal exam)
- amniocentesis (needle aspiration of amniotic fluid)
- arthrocentesis (needle aspiration of joint fluid)
- lumbar puncture (spinal tap to culture cerebrospinal fluid)
- paracentesis (abdominal tap via needle aspiration of abdominal fluid)
- thoracentesis/thoracocentesis (needle aspiration of the pleural cavity or lung abscess)