Write a short note on urinary tract infection and acute pyelonephritis.

Definition and Classification

Urinary tract infection (UTI) is the presence of micro-organisms in the urinary tract causing clinical illness, conventionally with significant bacteriuria (≥ 10⁵ colony-forming units/mL) — although lower counts are significant in symptomatic young women and in catheter or suprapubic specimens.

Aetiology

• *Escherichia coli* — 75–85% of uncomplicated infections, with uropathogenic strains possessing P fimbriae that adhere to uroepithelium.
• *Klebsiella*, *Proteus* (which produces urease, alkalinises the urine and forms STRUVITE STAGHORN CALCULI), *Enterococcus*, *Pseudomonas* (in complicated and catheter-associated infection), *Staphylococcus saprophyticus* (young sexually active women).
• *Candida* in diabetics, catheterised and immunosuppressed patients.
• Tuberculosis — consider in sterile pyuria (see below), which is of particular importance in India.

Predisposing factors: female sex (a short urethra and its proximity to the anus), sexual activity, pregnancy, diabetes mellitus, urinary stasis and obstruction (stones, prostatic enlargement, stricture, neurogenic bladder), catheterisation and instrumentation, vesicoureteric reflux, postmenopausal oestrogen deficiency, and immunosuppression.

Clinical Features

Cystitis (lower UTI): dysuria, frequency, urgency, suprapubic pain, strangury, haematuria, and cloudy offensive urine. Fever is absent or low grade, and the patient is not systemically unwell.

Acute pyelonephritis (upper UTI):

• High fever with rigors and systemic toxicity
• Loin (flank) pain with renal angle tenderness — the cardinal sign
• Nausea, vomiting, malaise
• Lower urinary tract symptoms may or may not be present
• Complications: septic shock, acute kidney injury, renal or perinephric abscess, emphysematous pyelonephritis (a necrotising gas-forming infection, occurring almost exclusively in diabetics and carrying high mortality), and papillary necrosis (in diabetes, sickle cell disease and analgesic nephropathy).

In the elderly, presentation is frequently atypical — confusion, falls, incontinence or simply "off legs", often without fever or urinary symptoms.

In children, features are non-specific — fever, vomiting, poor feeding, irritability and failure to thrive; UTI in a child requires investigation for vesicoureteric reflux and structural abnormality, since recurrent infection causes reflux nephropathy and renal scarring.

Investigations

• Urine dipstick — nitrites (specific, produced by Gram-negative organisms) and leucocyte esterase (sensitive). A negative dipstick in a low-probability patient makes UTI unlikely; a positive dipstick alone is insufficient in complicated cases.
• Urine microscopy — pyuria (> 10 white cells/µL), bacteriuria, red cells; WHITE CELL CASTS indicate PYELONEPHRITIS (renal parenchymal involvement) rather than cystitis.
• Urine culture and sensitivity — a clean-catch midstream sample before antibiotics. Essential in complicated UTI, pyelonephritis, pregnancy, men, children, recurrent infection and treatment failure; not required in a straightforward uncomplicated cystitis in a young woman.
• Blood tests in pyelonephritis: CBC, CRP, renal function, electrolytes, blood glucose, and blood cultures.
• Imaging — indicated in men, children, recurrent infection, suspected obstruction, failure to respond within 72 hours, and suspected abscess: ultrasound first, then CT (the investigation of choice for abscess, emphysematous pyelonephritis and obstruction); DMSA scan for scarring and MCUG for reflux in children.

STERILE PYURIA — an important differential: pyuria with a negative routine culture. Causes include partially treated UTI (the commonest), GENITOURINARY TUBERCULOSIS, fastidious organisms (*Chlamydia*, *Ureaplasma*, gonococcus), interstitial nephritis, analgesic nephropathy, papillary necrosis, stones, tumour, and adjacent inflammation (appendicitis). In India, sterile pyuria must always prompt consideration of renal tuberculosis, investigated with three early-morning urine samples for AFB, culture and CBNAAT.

Management

Uncomplicated cystitis in women: a short course (3 days) of an appropriate oral antibiotic — nitrofurantoin, fosfomycin or co-trimoxazole, guided by local resistance. Avoid fluoroquinolones as first-line for simple cystitis, given resistance and adverse effects. Advise fluids and analgesia.

Complicated UTI and UTI in men: 7–14 days of therapy, guided by culture; investigate for an underlying abnormality; consider prostatic involvement in men (requiring an agent with good prostatic penetration and a longer course).

Acute pyelonephritis:

• Mild, able to take orally: outpatient oral therapy with close follow-up.
• Systemically unwell, vomiting, pregnant, or complicated: admit; intravenous antibiotics (third-generation cephalosporin or aminoglycoside, adjusted to culture) with fluids and analgesia, switching to oral therapy on improvement, for a total of 10–14 days.
• Failure to improve within 48–72 hours demands imaging to exclude obstruction, abscess or emphysematous change — obstruction with infection is a urological emergency requiring urgent drainage (nephrostomy or stent), since antibiotics alone will not sterilise an obstructed system.
• Emphysematous pyelonephritis: aggressive resuscitation, glycaemic control, antibiotics, and percutaneous drainage, with nephrectomy in severe cases.

Special situations:

• PREGNANCY: asymptomatic bacteriuria MUST be screened for and TREATED, because it progresses to pyelonephritis in up to 30%, with risks of preterm labour and low birth weight. Use pregnancy-safe agents (nitrofurantoin — avoided at term; cephalosporins); avoid trimethoprim in the first trimester and fluoroquinolones throughout.
• Catheter-associated UTI: treat only if symptomatic; change or remove the catheter; do not treat asymptomatic bacteriuria.
• Recurrent UTI: behavioural measures (adequate fluids, voiding after intercourse, avoiding spermicides), topical vaginal oestrogen in postmenopausal women, and antibiotic prophylaxis (continuous low-dose or post-coital) in selected patients; investigate for structural causes. Cranberry products and methenamine have modest evidence.