Urinary Tract Infection in Adults
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Direct answer
Urinary tract infection in adults is bacterial infection of the urinary tract, with cystitis confined to the bladder and pyelonephritis involving the kidney. Escherichia coli causes most episodes, and a midstream urine culture with at least 10 to the power 5 colony-forming units per mL is the classical threshold for significant bacteriuria, though lower counts can matter in symptoms. Uncomplicated cystitis responds to short courses of nitrofurantoin, fosfomycin or trimethoprim-based therapy, while pyelonephritis needs longer treatment.
What you must remember
- Uncomplicated versus complicated: uncomplicated infection means a non-pregnant premenopausal woman without structural or urological abnormality; men, pregnancy and catheters count as complicated.
- Cystitis and pyelonephritis: dysuria, frequency and suprapubic discomfort suggest cystitis, while fever, loin pain and renal angle tenderness indicate pyelonephritis.
- Empirical choice: nitrofurantoin or fosfomycin for uncomplicated cystitis, and trimethoprim-sulfamethoxazole only where local resistance is low.
- Fluoroquinolones: reserved for severe pyelonephritis or when alternatives fail, because of tendon, aortic and neuropsychiatric risks.
- Asymptomatic bacteriuria: not treated in most non-pregnant adults, but screened and treated in pregnancy and before urological procedures that involve mucosal bleeding.
- ESBL organisms: ESBL-producing Escherichia coli is common in many Indian hospital settings, and carbapenems may be needed for severe sepsis, guided by the local antibiogram.
- Men and recurrence: urinary infection in men needs longer treatment and evaluation of the prostate, while recurrent cystitis in women may be prevented with methenamine or post-coital antibiotics.
Two adults with different urinary infections
The table sets two adults with urinary symptoms side by side, because the site of infection and the host decide the drug and duration.
| Feature | 28-year-old non-pregnant woman | 60-year-old man |
|---|---|---|
| Presentation | Two days of dysuria and frequency, without fever or loin pain | Fever, rigors and right loin pain |
| Working diagnosis | Uncomplicated cystitis | Pyelonephritis with possible sepsis; complicated infection is likely because of male sex and age |
| Investigations | Urine dipstick shows leucocytes and nitrites; culture sent if symptoms persist or local resistance is unknown | Blood and urine cultures; ultrasound excludes obstruction or an abscess |
| Antibiotic approach | Nitrofurantoin or fosfomycin for a short course, with advice on fluids | Intravenous ceftriaxone or similar, adjusted for ESBL risk and narrowed on the culture result, then oral step-down |
| Duration and follow-up | Short course | About one to two weeks in total, with a check for prostatic involvement if the response is slow |
| Key point | Nitrofurantoin suits lower tract infection | Nitrofurantoin would be inappropriate, because it gives poor tissue levels |
Where the examiner tries to catch you
Examiners return to the same few confusions in urinary tract infection.
- Trap: choosing nitrofurantoin for pyelonephritis because it is the familiar cystitis drug Correct: it reaches high concentrations in urine but not in renal tissue or blood, so it is only for lower tract infection, and fever with loin pain must steer you elsewhere.
- Trap: reading a colony count without checking how the sample was taken Correct: a count of 10 to the power 5 per mL is the threshold for a clean midstream sample, while catheter specimens and suprapubic aspirates are interpreted at lower thresholds.
- Trap: calling pyuria an infection Correct: sterile pyuria suggests tuberculosis, stones or interstitial nephritis.
- Trap: treating asymptomatic bacteriuria in an elderly or catheterised patient Correct: it is usually left alone, and the examiner expects you to say so.
Frequently asked questions
What colony count confirms significant bacteriuria in a symptomatic adult woman?
A count of at least 10 to the power 5 colony-forming units per mL in a clean midstream urine sample confirms significant bacteriuria in a symptomatic adult woman. Catheter specimens and suprapubic aspirates are interpreted at lower thresholds, so the sampling method must be considered before reading the count.
Why is nitrofurantoin not used for pyelonephritis?
Nitrofurantoin is not used for pyelonephritis because it reaches high concentrations in the bladder urine but achieves poor levels in renal tissue and the bloodstream. It is therefore reserved for lower urinary tract infection, while pyelonephritis needs an agent that penetrates the kidney and reaches therapeutic blood levels.
When should asymptomatic bacteriuria be treated in adults?
Asymptomatic bacteriuria in adults should be treated in pregnancy, where it reduces the risk of pyelonephritis and preterm birth, and before urological procedures that involve mucosal bleeding. In most other non-pregnant adults, including the elderly and catheterised patients, it is usually left untreated because treatment does not improve outcomes and promotes resistance.
Which organism causes most urinary tract infections in adults?
Escherichia coli causes most urinary tract infections in adults, accounting for the large majority of uncomplicated episodes. Other organisms such as Klebsiella, Proteus and Enterococcus are more common in complicated infection, catheterisation and hospital-acquired disease, and local antibiograms guide empirical treatment.
What is the first-line treatment for uncomplicated cystitis in non-pregnant women?
Nitrofurantoin or fosfomycin is the first-line treatment for uncomplicated cystitis in non-pregnant women, given for a short course that depends on the drug chosen. Trimethoprim-sulfamethoxazole is an option only where local resistance is known to be low, and the choice should be guided by the local antibiogram.