A lab MIC test tells you the lowest antibiotic concentration that stops a specific strain's growth in a dish — it says nothing about whether that concentration is ever actually reached where the infection lives in a real patient. The concentration that shows up at the infection site depends on the prescribed dose and how well that particular tissue lets the drug through.
achievable = peak_serum_conc × dose_multiplier × site_penetration
verdict = achievable > MIC ? clears : persists
- MIC — a genuine, measured property of this strain against this drug; lower means more susceptible in vitro.
- Dose — raises serum concentration everywhere, but proportionally — it can't fix a bad penetration factor on its own.
- Site penetration — urine concentrates the drug far above serum (great for UTIs); skin/soft tissue gets a moderate fraction; the blood-brain barrier lets very little through, so even a "susceptible" low-MIC strain can survive there.
Clinical relevance: this is exactly why a bacteremia and a meningitis caused by the identical, identically-susceptible strain can need different antibiotics or much higher doses for the CNS infection — the lab report alone never tells the whole story.