From the FDA Drug Label
CLINICAL PHARMACOLOGY 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 Studies have shown that following intravenous administration of cefazolin to normal volunteers, mean serum concentrations peaked at approximately 185 mcg/mL and were approximately 4 mcg/mL at 8 hours for a 1 gram dose. Human Pharmacology Cephalexin is acid stable and may be given without regard to meals. It is rapidly absorbed after oral administration. Following doses of 250 mg, 500 mg, and 1 g, average peak serum levels of approximately 9,18, and 32 mcg/mL, respectively were obtained at 1 hour.
The bioavailability of IV Ancef (cefazolin) and oral Keflex (cephalexin) can be compared by looking at their peak serum concentrations.
- IV Ancef: peak serum concentration of approximately 185 mcg/mL at 1 hour for a 1 gram dose 1
- Oral Keflex: peak serum concentration of approximately 32 mcg/mL at 1 hour for a 1 gram dose 2 Since the peak serum concentration of IV Ancef is higher than that of oral Keflex, the bioavailability of IV Ancef is higher than that of oral Keflex.
From the Research
Intravenous Ancef (cefazolin) has 100% bioavailability compared to oral Keflex (cephalexin) which has approximately 90% bioavailability. This difference occurs because IV administration delivers medication directly into the bloodstream, bypassing the digestive system entirely. Oral Keflex must be absorbed through the gastrointestinal tract, where some drug may be lost due to incomplete absorption or first-pass metabolism in the liver.
Key Points to Consider
- The choice between IV Ancef and oral Keflex depends on the clinical situation, with IV Ancef preferred for more severe infections, hospitalized patients, or when rapid drug delivery is needed, while oral Keflex is suitable for less severe infections in outpatient settings 3.
- Both are first-generation cephalosporins with similar antimicrobial coverage against gram-positive organisms and limited gram-negative coverage.
- A recent meta-analysis comparing cefazolin to other antibiotics for surgical site infection prevention found that cefazolin is as effective as other agents, such as cefuroxime, ceftriaxone, and cefamandole, in preventing surgical site infections 4.
Clinical Implications
- Keflex is typically dosed at 250-500mg orally every 6 hours for most infections, while Ancef is usually given at 1-2g IV every 8 hours, depending on infection severity and site.
- The bioavailability of oral Keflex is approximately 90%, which is lower than that of IV Ancef, but still effective for its indicated uses 5.
- The most recent and highest quality study suggests that cefazolin is a suitable choice for surgical prophylaxis due to its efficacy and safety profile 4.