From the FDA Drug Label
After intramuscular administration of cefazolin to normal volunteers, the mean serum concentrations were 37 mcg/mL at 1 hour and 3 mcg/mL at 8 hours following a 500 mg dose, and 64 mcg/mL at 1 hour and 7 mcg/mL at 8 hours following a 1 gram dose The serum half-life for cefazolin is approximately 1. 8 hours following IV administration and approximately 2 hours following IM administration.
The best bioavailable option for transition from IV Ancef (cefazolin) is Intramuscular (IM) administration.
- Dosing: 500 mg to 1 gram IM every 8 hours, depending on the severity of the infection and the patient's renal function.
- Key points:
- Mean serum concentrations after IM administration are lower than after IV administration, but still effective against most strains of microorganisms.
- The serum half-life after IM administration is approximately 2 hours, which is similar to the half-life after IV administration.
- IM administration is a suitable alternative to IV administration when the patient's condition allows for it 1.
From the Research
The best bioavailable oral option for transitioning from IV Ancef (cefazolin) is cephalexin (Keflex), typically dosed at 500 mg orally twice or thrice daily for adults with normal renal function, as evidenced by its excellent bioavailability and similar antimicrobial coverage to IV cefazolin 2. When transitioning, ensure the infection is responding clinically before switching from IV to oral therapy, as evidenced by improving symptoms, decreasing inflammatory markers, and hemodynamic stability.
- Key considerations for transitioning to oral cephalexin include:
- Ensuring the patient has a functioning gastrointestinal tract to absorb the medication
- Monitoring for signs of clinical improvement, such as reduced fever, improved respiratory symptoms, and decreased white blood cell count
- Adjusting the dose for patients with renal impairment: for creatinine clearance 10-50 mL/min, reduce to 500 mg every 8-12 hours; for clearance below 10 mL/min, give 500 mg every 12-24 hours
- The oral bioavailability of cephalexin is excellent at 90-100%, ensuring adequate drug concentrations at the infection site, as supported by studies examining the pharmacokinetics and pharmacodynamics of cephalexin 2.
- If the patient has a penicillin allergy, alternative antibiotics may be considered, though their spectrum may differ somewhat from cefazolin, and the choice should be guided by susceptibility testing and clinical judgment 3.