By Michael Spinner
When faced with a specific organism identified in culture (and especially if the antibiotic sensitivities are back from the lab!), it’s typically pretty straightforward to determine the appropriate antibiotic. A greater challenge arises when we DON’T know the causative organism(s) and have to think about empiric antibiotic coverage. Choosing empiric antibiotics can be tough as it often involves a delicate balance between providing broad-spectrum coverage while not being overly aggressive and breaking out imipenem all of the time! In this post, we’ll consider a few patient cases to practice choosing empiric antibiotics.
Case 1. A previously healthy 26-year-old medical student presents to student health clinic with 1 week of fever, productive cough, and pleuritic chest pain. Chest X-ray shows a right lower lobe consolidation.
This is a classic scenario for community-acquired pneumonia (CAP). The productive cough and consolidation in this case are most consistent with a “typical” pathogen like Streptococcus pneumoniae, but “atypical” bugs like Mycoplasma pneumoniae and Chlamydia pneumonia are also very common in this age group. Azithromycin has great coverage of both the typical and atypical organisms causing CAP making it a great choice for empiric coverage in this setting.
Case 2. A 65-year-old male with a history of HTN, HLD, T2DM, and hip OA is post-operative day 4 status-post right total hip replacement. Over the past 24 hours, he has developed worsening fever and productive cough. CBC shows a new leukocytosis and chest X-ray shows a lingular infiltrate.
This patient contracted pneumonia at least 48-72 hours after being admitted to the hospital and thus meets criteria for hospital-acquired pneumonia (HAP). For HAP, empiric antibiotics should cover both gram-positive and gram-negative organisms, the most important of which are MRSA and Pseudomonas aeruginosa. We’ve all seen vanc/Zosyn prescribed countless times on the wards and this is why! Vancomycin provides great coverage for MRSA and Zosyn (piperacillin/tazobactam) covers Pseudomonas. If the patient aspirated, Zosyn would still be an appropriate choice as it provides great anaerobic coverage as well. Pending culture results and antibiotic sensitivities, antibiotic coverage should be narrowed in this patient.
Case 3. A 37-year-old male with HIV/AIDS and CD4 count of 140 presents to the emergency department with fever, non-productive cough, and shortness of breath. He is hypoxic and a chest X-ray shows bilateral interstitial infiltrates.
This presentation is highly suspicious for Pneumocystis jirovecii pneumonia, and the patient should be treated empirically with trimethoprim/sulfamethoxazole and corticosteroids (classic Step 2 CK question!). If the patient lived in a region endemic for Histoplasma, Blastomyces, or Coccidioides, empiric anti-fungal coverage could also be considered. If the patient had a CD4 count <50, azithromycin, ethambutol, and rifampin might also be added as empiric coverage for atypical mycobacterial infection like Mycobacterium avium-intracellulare complex.
Note that in all three of the cases above, the patient presented with signs and symptoms of pneumonia. However, differences in the patient histories and risk factors led to important differences in the likely pathogens and empiric antibiotic coverage.
For more information on antibiotics, please reference the following PDFs for some high-yield points about the 16 antibiotics that you should know and antibiotic regimens for common infections. These cards might even make a good pocket reference for the wards!