Managing patients with bacterial infections is part of the bread and butter of internal medicine. Sometimes the infectious source is obvious, but other times no infection is apparent, despite vitals and lab tests suggestive of sepsis. Still other times, there are multiple sources of infection.
It is nice to have a framework for diagnosing and managing patients with bacterial infections.
General Principles for Suspected Bacterial Infections
- Name the infection as specifically as possible.
- Once this is done, look up the right empiric antibiotics (if you don’t know them).
- Use cultures to tailor antibiotic therapy.
- If the source of infection is not obvious, but you suspect bacterial infection, start broad-spectrum antibiotics (if you think the patient is sick). A usual choice for this is vancomycin and cefepime, with anaerobic coverage, such as adding metronidazole (if an intra-abdominal infection or other potential anaerobic infection is being considered). If the patient is clinically well, it may be acceptable to hold off starting antibiotics pending further work up or culture results.
Common Bacterial Infections and How to Test for Them
Start with a chest XR and consider a CT of the chest, if necessary.
Start with a UA and consider ultrasound of the kidneys or CT of the abdomen, if necessary.
Examine the skin! This can be tricky with elderly and obese patients. Enlist help from the nurses. Don’t forget to look at the back and under the socks. If necessary, order CT scan or ultrasound to look for deep infections or abscesses.
Examine relevant areas. Is there erythema around a dialysis catheter or tender swelling over an implanted pacemaker? MR imaging of implanted orthopedic hardware may be appropriate.
(Biliary system, colitis, intra-abdominal abscess, peritonitis, and more): Depending on physical exam findings and patient history, can consider RUQ US (right upper quadrant ultrasonography) or CT abdomen and pelvis.
Consider osteomyelitis at areas of spinal tenderness or associated with ulcers. MRI is the best test for this.
Order blood cultures.
Examine the joints for tenderness, swelling, and reduced range of motion. Obtain a sample of joint fluid if infection suspected.
Check for nuchal rigidity and photophobia, and do a neurologic exam. If these are suspected, do a lumbar puncture.
Obviously, this is an incomplete list. I heard about a case of mycobacterial invasion of the bone marrow after intra-vesicular BCG treatment for bladder cancer that took a long time to diagnose—the patient had many negative blood cultures and a negative whole body CT scan. The diagnosis was made with a culture of bone marrow aspirate. The initial reason for admission was hypercalcemia ultimately secondary to granulomas in the bone marrow. If initial testing doesn’t make things clear, the important thing is to stay curious.
For a patient who seems clinically septic but has no localized exam findings or clues on HPI, start by looking for the most common bacterial infections. A basic workup is:
- a physical exam,
- CXR,
- UA, and
- two sets of blood cultures.
If the patient is decompensating and no source is identified, often the next step is CT abdomen and pelvis.
When things aren’t clear, procalcitonin can be helpful in determining if a bacterial infection is present or not. Keep in mind that this is just one data point, and it should not be used by itself to give or withhold antibiotics.
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