Certain antibiotics are never used to treat urinary tract infections… — Medical Notes Dr. Ayman Radwan

Medical Notes Dr. Ayman Radwan@mednotesaymanR · 1.5K subscribers Open in TelegramOpen

Certain antibiotics are never used to treat urinary tract infections (UTIs)-even if a lab test says the bacteria is sensitive-because they do not achieve high enough concentrations in the urine to clear the infection including
(1)Moxifloxacin : Unlike other fluoroquinolones (like ciprofloxacin or levofloxacin), moxifloxacin is metabolized and excreted primarily by the liver, meaning it does not reach effective therapeutic levels in the urine
(2)Macrolides (Azithromycin, Clarithromycin): While excellent for lung or soft-tissue infections, these do not accumulate well in the bladder or urinary tract in amounts needed to kill bacteria
(3) clindamycin : Only about 10% of the active drug is excreted in the urine
(4) Doxycycline: Unlike older tetracyclines"Tetracycline and minocycline" , it is excreted primarily via the feces and intestines, then its urinary concentrations are much lower
(5) Linezolid : while it has higher tissue penetration than vancomycin (eg for pneumonia and infective endocarditis ) and even good efficacy againt intracellular staph and dormant state.it has poor urinary concentrations
Approximately 30-35% of a Linezolid dose is excreted into the urine as active, unchanged drug. The majority of the drug (roughly (50 % ) is converted via hepatic oxidation into two primary, inactive metabolites (aminoethoxyacetic acid and hydroxyethyl glycine)
While glomerular filtration helps excrete Linezolid, the drug also undergoes significant renal tubular reabsorption. This reabsorption slows overall drug clearance and contributes to the low levels of active drug remaining in the urine
(6) Tigecycline (Tygacil) : Only about 22% of a tigecycline dose is excreted unchanged in the urine, with the vast majority eliminated via the biliary and fecal routes
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in the following episode we will discuss the reverse , antibiotics than can sometimes treat lower urinary tract infections (UTIs) despite culture reports indicating "resistance" due to supratherapeutic urinary concentration
(Augmentin -Nitrofurantoin-Cephalosporins ( Ceftriaxone, Cephalexin)-Fosfomycin +/-Fluoroquinolones )
this was discussed in details in our paper
https://doi.org/10.1007/s40121-026-01361-0
also we discussed earlier the effect of PH on activation /inactivation of antibiotics "including acidic Vs alkaline unrine"
-Staph. sapro : the second most common cause of UTI in general after E.coli can cause alkaline urine
you have to check if your choice of antibiotics is less effective in alkaline urine (Nitrofurantoin,Tetracyclines and Most Beta-lactams)
NB: amoxicillin-clavulanic acid and imipenem, are largely unaffected by pH
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As we always say it is a whole complex dynamic story rather than the ridiculously simplified static MIC and sensitivity result in vitro !