What it is:A urinary tract infection is microorganisms in the urine that can't be explained by contamination, with the potential to invade the bladder, urethra, prostate, or kidney. That single definition covers a spectrum from a nuisance case of cystitis to a patient who is septic from pyelonephritis.
The core problem:Symptoms alone can't tell you whether you're dealing with a 3-day problem or a 6-week problem. A 25-year-old with dysuria and a 70-year-old man with the exact same organism need completely different workups, drugs, and durations, and picking the wrong lane is the most common way students get UTI questions wrong.
What you do about it:Sort the patient into a lane first (uncomplicated cystitis vs. pyelonephritis vs. complicated vs. male vs. prostatitis), then match the drug to the site. Cystitis just needs high urine concentrations for a few days. Pyelonephritis and prostatitis need drugs that actually penetrate kidney and prostate tissue, for weeks, because that's where the bacteria actually are.
Every decision in this chapter collapses to three questions: where is the infection(bladder vs. kidney vs. prostate), is it complicated(any structural, functional, or host factor that impairs normal urine flow or defenses), and is the patient male(if yes, treat it as complicated by default and get a culture, full stop). Answer those three and the drug and duration almost pick themselves.
UTI is really an umbrella term. Urethritis, cystitis (bladder), prostatitis (prostate), and epididymitis are lower tract infections. Pyelonephritis (kidney) is the only upper tract infection.That anatomic split is the whole reason duration and drug selection change so much between "just cystitis" and "it's in the kidney now."
| Category | Definition | Why it matters |
|---|---|---|
| Uncomplicated | No structural or functional abnormality interfering with urine flow or voiding | Predictable organism (E. coli), predictable susceptibility, short empiric course without a culture |
| Complicated | A predisposing lesion is present: congenital abnormality, stone, indwelling catheter, prostatic hypertrophy, obstruction, or neurologic deficit affecting urine flow | Wider range of organisms, higher resistance, culture-directed therapy, longer course |
| Recurrent | ≥2 UTIs in 6 months, or ≥3 in 1 year | Splits into reinfection (different organism, most common) vs. relapse (same organism) - the two have completely different workups |
Reinfectionis caused by a different organism and accounts for the majority of recurrent UTIs; it's often behavioral (sexual activity, spermicide use) and gets managed with prophylaxis or per-episode treatment. Relapseis the same organism coming back, usually because the original course didn't fully sterilize the tissue, a stone or abscess is harboring bacteria, or the drug never penetrated the source well enough. Relapse is the one that earns a urologic workup, not just a repeat round of antibiotics.
Almost every UTI-causing organism starts as normal bowel flora. Bacteria reach the urinary tract by three routes: ascending(by far the dominant route, and the reason women get UTIs so much more often, since the urethra is short and close to the rectum), hematogenous/descending, and lymphatic.
| Setting | Typical organisms |
|---|---|
| Uncomplicated, community-acquired | E. coli(>80-90%), Staphylococcus saprophyticus, Klebsiella pneumoniae, Proteusspp., Pseudomonas aeruginosa, Enterococcusspp. |
| Complicated or nosocomial | E. colidrops to <50% of isolates; more Proteus, Klebsiella, Enterobacter, P. aeruginosa, staphylococci. Enterococcusis the second most common organism in hospitalized patients. |
Most UTIs are caused by a single organism. Multiple organisms showing up on culture is itself a clinical clue, it points toward stones, indwelling catheters, or a chronic renal abscess rather than routine cystitis. Vancomycin-resistant enterococci (VRE)are increasingly seen in patients with long hospitalizations or underlying malignancy, which matters when you're building empiric coverage for a sick, hospitalized, complicated patient.
Uncomplicated cystitis is dominated by one predictable organism with predictable local susceptibility, so it's cost-effective to treat empirically off a urinalysis alone, without waiting on a culture. Complicated and nosocomial infections pull from a much wider, less predictable organism pool that includes Pseudomonasand enterococci, so empiric coverage has to broaden and a culture becomes essential rather than optional.
| Tract | Symptoms |
|---|---|
| Lower UTI | Dysuria, urgency, frequency, nocturia, suprapubic heaviness, gross hematuria, costovertebral tenderness |
| Upper UTI (pyelonephritis) | Flank pain, fever, nausea, vomiting, malaise |
The only way to actually distinguish contamination from infection is demonstrating significant numbers of organisms in a properly collected specimen. That's why a urinalysis, and often a culture, is part of the workup even when the story sounds classic.
Older adults are the classic trap.They frequently skip the textbook dysuria/frequency picture entirely and instead present with altered mental status, a change in eating habits, or nonspecific GI symptoms. If you're only screening elderly patients for "the usual" UTI symptoms, you'll miss a real infection sitting behind a vague presentation.
Start with a standard urinalysis on every patient. Microscopic exam via Gram stain of an unspun or centrifuged specimen is the fastest window into what you're dealing with.
| Test | What it tells you |
|---|---|
| Gram stain, oil-immersion field | ≥1 organism/field on a properly collected, uncentrifuged specimen correlates with >105CFU/mL of urine |
| Quantitative colony count | ≥105CFU/mL is the classic threshold for a UTI, but up to 50% of symptomatic femalespresent with lower counts (as low as 103CFU/mL) - the cutoff misses real, symptomatic infection nearly half the time |
| Pyuria | >10 WBC/mm3in a symptomatic patient correlates with significant bacteriuria; 5-10 WBC/mm3is the accepted upper limit of normal |
| Nitrite test | Detects nitrate-reducing organisms (e.g., E. coli); dipstick, fast |
| Leukocyte esterase | Rapid dipstick surrogate for pyuria |
| Quantitative urine culture | The most reliable diagnostic method. >105bacteria/mL is typical, but as many as one-third of symptomatic femalesculture below that threshold |
A negative nitrite test doesn't rule out infection. It only detects nitrate-reducing organisms, so infections caused by non-nitrate reducers like S. saprophyticusor Enterococcuswill read falsely negative. Pair it with clinical judgment and pyuria, don't lean on it alone.
Goals of therapy:eradicate the invading organism, prevent or treat systemic consequences, prevent recurrence, and minimize collateral damage from unnecessarily broad-spectrum antimicrobials. Selection hinges on severity of presentation, site of infection, complicated vs. uncomplicated status, local susceptibility, side-effect profile, cost, recent antibiotic exposure, and how convenient the regimen actually is for the patient to complete.
Because E. colidominates and its susceptibility is generally predictable, the cost-effective approach is a urinalysis plus empiric therapy, without a routine urine culture. Short-course therapy (3 days) beats single-dose therapyfor uncomplicated infection, with one deliberate exception: fosfomycin is designed as a single dose and stays effective as one.
Nitrofurantoin, TMP-SMX, and fosfomycin are first-linefor uncomplicated cystitis. Fluoroquinolones work too, but they're deliberately held in reserve for suspected or possible pyelonephritisbecause of their collateral damage risk (C. difficile, tendon injury, resistance selection pressure on other organisms). Don't reach for a fluoroquinolone just because it "covers everything" in straightforward cystitis, that's the wrong instinct here.
Most E. coliremain TMP-SMX susceptible, but resistance is climbing and has been reported as high as 27% in some areas. If local E. coliresistance to TMP-SMX exceeds 20%, switch to nitrofurantoin or fosfomycin instead.β-lactams as a class are less effective than TMP-SMX or fluoroquinolones for acute cystitis, and amoxicillin or ampicillin alone should not be used given resistance rates.
| Indication | Drug | Oral dose | Interval | Duration |
|---|---|---|---|---|
| Uncomplicated cystitis | Trimethoprim-sulfamethoxazole | 1 DS tablet | Twice daily | 3 days |
| Nitrofurantoin monohydrate | 100 mg | Twice daily | 5 days | |
| Fosfomycin trometamol | 3 g | Single dose | 1 day | |
| Ciprofloxacin | 250 mg | Twice daily | 3 days | |
| Levofloxacin | 250 mg | Once daily | 3 days | |
| Amoxicillin-clavulanate | 500 mg | Every 8 hours | 5-7 days | |
| Complicated cystitis | Trimethoprim-sulfamethoxazole | 1 DS tablet | Twice daily | 7-10 days |
| Ciprofloxacin | 250-500 mg | Twice daily | 7-10 days | |
| Levofloxacin | 250 mg (or 750 mg) | Once daily | 10 days (or 5 days at 750 mg) | |
| Amoxicillin-clavulanate | 500 mg | Every 8 hours | 7-10 days | |
| Recurrent infection prophylaxis | Nitrofurantoin | 50 mg | Once daily | 6 months |
| Trimethoprim-sulfamethoxazole | 1/2 SS tablet | Once daily | 6 months | |
| Acute pyelonephritis | Trimethoprim-sulfamethoxazole | 1 DS tablet | Twice daily | 14 days |
| Ciprofloxacin | 500 mg (or 1000 mg ER) | Twice daily (ER once daily) | 14 days (ER: 7 days) | |
| Levofloxacin | 250 mg (or 750 mg) | Once daily | 10 days (or 5 days at 750 mg) | |
| Amoxicillin-clavulanate | 500 mg | Every 8 hours | 14 days |
Same drug, same drug family, but the duration climbs as the infection moves from bladder to kidney to complicated disease: 3 days for simple cystitis, 7-10 days once it's complicated, 14 days once it's in the kidney. Dose intervals stay mostly the same, duration is what does the work.
Highly effective against most aerobic enteric bacteria except P. aeruginosa. Achieves high urinary tract tissue concentrations, which is exactly why it holds up in complicated infections and why it's also used as long-term prophylaxis for recurrent UTIs, not just acute treatment.
Rash, including Stevens-Johnson syndrome, renal failure, photosensitivity, and hematologic toxicity (neutropenia, anemia). Monitor serum creatinine, BUN, electrolytes, rash, and CBC.
Local resistance matters here more than almost any other UTI drug decision: reported resistance up to 27% in some regions means you cannot assume it will work without knowing your local antibiogram.
Effective both therapeutically and prophylactically in recurrent UTIs. Its main advantage is durable low resistance, even after long courses of therapy, which is unusual and part of why it's still first-line despite decades of use.
Adverse effects:GI intolerance, neuropathies, and pulmonary reactions. Get a baseline serum creatinine and BUN.
It only achieves meaningful concentrations in the bladder, not the kidney or prostate, which is exactly why it has no role in pyelonephritis or prostatitis.
Single-dose therapy for uncomplicated infections, with low levels of resistance. Adverse effects: diarrhea, headache, angioedema. No routine monitoring tests are recommended. Use with caution in hepatic dysfunction.
Greater spectrum of activity, including P. aeruginosa, and effective for both pyelonephritis and prostatitis because they penetrate tissue well. Avoid in pregnancy and children.Moxifloxacin should not be usedfor UTI, it doesn't achieve adequate urinary concentrations despite being a fluoroquinolone.
Hypersensitivity, photosensitivity, GI symptoms, dizziness, confusion, and tendonitis/tendon rupture (black box warning). Monitor CBC, baseline serum creatinine, and BUN.
Rising E. coliresistance to plain aminopenicillins has made amoxicillin-clavulanate the preferred penicillin for uncomplicated cystitis(plain amoxicillin/ampicillin alone should not be used). Adverse effects: hypersensitivity (rash, anaphylaxis), diarrhea, superinfection, seizures. Monitor CBC and for rash/hypersensitivity.
No major advantage over other first-line agents, more expensive, and not active against enterococci. Same monitoring as other β-lactams: CBC, signs of rash/hypersensitivity.
Aminoglycosides (gentamicin, tobramycin, amikacin):renally excreted, achieve good urinary concentrations. Amikacin is generally reserved for multidrug-resistant organisms. Toxicities: ototoxicity, nephrotoxicity. Monitor serum creatinine, BUN, and individualized pharmacokinetics.
Extended-spectrum penicillins (ampicillin-sulbactam, piperacillin-tazobactam):more active against P. aeruginosaand enterococci than cephalosporins, and useful when you want to avoid an aminoglycoside or the patient has renal impairment.
Parenteral cephalosporins:second- and third-generation agents (ceftriaxone, ceftazidime, cefepime) have broad gram-negative activity but no enterococcal coverage. Ceftazidime and cefepime cover P. aeruginosa; useful for nosocomial infection and urosepsis.
Carbapenems/monobactams:imipenem, meropenem, and doripenem cover P. aeruginosaand enterococci, but ertapenem does not. Aztreonam is gram-negative only, useful in penicillin-allergic patients. Generally reserved for nosocomial infection when aminoglycosides need to be avoided.
High-grade fever (>38.3°C / 100.9°F) plus severe flank pain should be treated as pyelonephritis, and it warrants aggressive management. At presentation: Gram stain of urine, urinalysis, culture, and sensitivities, every time.
| Severity | Setting | Approach |
|---|---|---|
| Mild-to-moderate | Outpatient, oral | Fluoroquinolone (cipro or levo) orally x 7-10 days is first-line.TMP-SMX x 14 days is an alternative. If Gram stain shows gram-positive cocci, think Enterococcus faecalisand direct therapy at it (ampicillin). |
| Severely ill | Hospitalize, IV | IV fluoroquinolone, an aminoglycoside with or without ampicillin, or an extended-spectrum cephalosporin with or without an aminoglycoside |
If the patient was hospitalized in the last 6 months, has a urinary catheter, or lives in a nursing home, think P. aeruginosa, enterococci, and multidrug-resistant organisms. That triggers broad-spectrum empiric therapy: an extended-spectrum β-lactam/β-lactamase inhibitor or a carbapenem.
Obtain a follow-up urine culture 2 weeks after completing therapyto confirm a satisfactory response and catch relapse early.
D.S., a 36-year-old otherwise healthy woman with 4 days of dysuria plus 2 days of chills, fever, and flank pain, is diagnosed with pyelonephritis. Baseline labs are normal, she's not pregnant, and she's allergic to sulfa. That allergy takes TMP-SMX off the table immediately, which is exactly the kind of detail that should reroute you to an oral fluoroquinolone for mild-to-moderate outpatient management. When her culture returns 4+ E. coliand she isn't improving by day 2, that's a signal to reassess (adequate dosing, adherence, possible need for IV therapy or hospitalization) rather than just waiting it out. Once she's afebrile 36-48 hours and clinically improving, she can step down to an oral regimen to complete the full course as an outpatient. That IV-to-PO, culture-directed narrowing is the core workflow for any hospitalized pyelonephritis patient, not just this one.
Therapy in men requires prolonged treatment, and the reasons trace straight back to anatomy: the prostate is a reservoir that ordinary short courses don't sterilize.
| Step | Detail |
|---|---|
| Culture first | Always obtain a urine culture before treating; the causative organism is far less predictable in men than in women |
| Empiric choice | If gram-negative bacteria are presumed, TMP-SMX or a fluoroquinolone |
| Initial duration | 10-14 days |
| Recurrent infection in men | Cure rates are much higher with a 6-week TMP-SMX regimenthan with a standard short course |
The men's management pathway branches on whether there's a prostatic source: a complicated but non-prostatic infection gets treated for 2 weeks with a follow-up culture; signs of acute prostatitis or pyelonephritis route the patient to 6 weeks of prostatic-source therapy, sometimes starting with 2 weeks of hospitalization and parenteral antibiotics before the oral phase. Persistent positive cultures after the prostatic course prompt consideration of long-term suppression or surgery.
| Pathogens | Recommended therapy | Comments |
|---|---|---|
| E. coli, K. pneumoniae, Proteusspp., P. aeruginosa | TMP-SMX x 4-6 weeks, or a fluoroquinolone x 4-6 weeks | Acute prostatitis may require IV therapy initially. Chronic prostatitis may require longer treatment periods or surgery. |
The prostate is a pharmacologic reservoir that most antibiotics can't reach in useful concentrations. TMP-SMX and the fluoroquinolones are the agents that actually achieve therapeutic prostatic tissue levels, which is exactly why they show up here and in the fluoroquinolone drug summary as "effective for prostatitis" while nitrofurantoin, which never leaves the bladder, has no role at all. The extended duration exists because you're trying to sterilize tissue that's hard to penetrate in the first place, not because the organism is unusually resistant.
Recurrent UTIs (reinfections and relapses together) make up a significant share of all UTIs, and they're overwhelmingly a female problem. Split patients into two groups based on frequency.
In women whose symptomatic reinfections track with sexual activity, voiding after intercoursecan help mechanically clear organisms before they establish infection, and self-administered, single-dose TMP-SMX taken after intercoursesignificantly reduces recurrence. This is a distinct strategy from daily suppressive prophylaxis, matched to a distinct trigger.
Relapse gets escalating durations, not a repeat of the same course:relapse after short-course therapy earns a 2-week course; relapse after that 2-week course extends therapy another 2-4 weeks; and relapse after 6 weeks of treatment triggers a urologic examination, with therapy for 6 months or longer on the table.
Treat both symptomatic and asymptomatic bacteriuriain pregnancy to avoid downstream pregnancy complications. Use a low-adverse-effect agent, typically cephalexin, amoxicillin, or amoxicillin/clavulanate for 7 days.
Tetracyclines(teratogenic). Sulfonamides in the third trimester(risk of kernicterus and hyperbilirubinemia in the newborn). Fluoroquinolones, entirely, throughout pregnancy (potential to inhibit fetal cartilage and bone development).
| Scenario | Approach |
|---|---|
| Asymptomatic bacteriuria, short-term catheter (<30 days) | Withhold systemic antibiotics. Remove the catheter as soon as possible. |
| Symptomatic, short-term catheter | Remove the catheter again, and treat as a complicated infection. |
| Long-term catheterization | Antibiotics only postpone bacteriuria and select for resistant organisms; asymptomatic bacteriuria is not treated here either. |
Positive urine culture in an asymptomatic catheterized patient is not, by itself, a reason to prescribe antibiotics. Overtreating it doesn't help the patient and actively breeds resistant organisms. This is one of the most common inappropriate-antibiotic-use scenarios in inpatient settings.
| Parameter | When | Watching for |
|---|---|---|
| Symptom resolution | 48-72 hours after starting therapy | Clinical improvement; lack of improvement should prompt reassessment of the regimen, adherence, or need for escalation |
| Follow-up urine culture | 2 weeks post-therapy for pyelonephritis, complicated infection, and infections in men | Relapse vs. cure; positive culture routes to retreatment and possible urologic workup |
| Serum creatinine, BUN | Baseline, especially with TMP-SMX, nitrofurantoin, fluoroquinolones, or aminoglycosides | Renal function, dose adjustment needs |
| CBC / signs of rash | Baseline and during therapy with TMP-SMX, β-lactams, or fluoroquinolones | Hematologic toxicity, hypersensitivity reactions |
| Serum aminoglycoside concentrations | During IV aminoglycoside therapy | Individualized pharmacokinetic dosing; ototoxicity and nephrotoxicity avoidance |
| Monthly urine culture | Throughout a 6-month prophylactic course for recurrent infections | Breakthrough infection despite prophylaxis |