Clinical presentations and diagnosis of acute pyelonephritis

Definition and organisms. Acute bacterial infection of the renal pelvis and parenchyma, usually arising by ascending spread from the bladder and, less often, haematogenously. Urinary infection is among the commonest infections of older people, and Pathy's emphasises that its pathogenesis is strongly related to obstructive uropathy. Escherichia coli predominates, but Kane's table shows that in older adults Proteus, Klebsiella, Enterobacter, Pseudomonas and Enterococcus are much more frequent than in the young — and in India, ESBL-producing and carbapenem-resistant organisms are now common.

Predisposing factors in the elderly. Obstructive uropathy — benign prostatic hyperplasia in men, pelvic organ prolapse and cystocele in women; incomplete bladder emptying with a raised post-void residual; catheterisation and instrumentation; stones; diabetes; vaginal and urethral atrophy; immobility and bed rest with urinary stasis; neurogenic bladder; and previous urological surgery. The age-related fall in GFR of about 30% and reduced renal reserve mean acute kidney injury occurs readily.

Classic clinical presentation. Fever with rigors, loin or flank pain, costovertebral angle tenderness, nausea and vomiting, with or without lower urinary tract symptoms (dysuria, frequency, urgency, suprapubic discomfort). Gram-negative bacteraemia and septic shock may develop abruptly and be life-threatening — a point Pathy's makes explicitly.

Presentation in the older adult — the examinable half of the question. It is frequently atypical and non-localising. Delirium or acute confusion, new or worsening urinary incontinence, falls, functional decline, anorexia, generalised weakness and tachypnoea may be the only features. Fever is blunted or absent in a substantial proportion, and Kane's warns that hypothermia should be treated as sepsis until proved otherwise; the absence of leucocytosis does not exclude bacterial infection. Loin pain and costovertebral tenderness are often absent. The necessary counterbalance: do not attribute delirium to the urine unless there are localising urinary symptoms or systemic sepsis with no other explanation, because asymptomatic bacteriuria is very common and must not be treated.

Investigations. Urinalysis — pyuria, nitrites, leucocyte esterase, and white cell casts, which indicate renal parenchymal involvement. Urine culture with sensitivity must be sent before antibiotics in every case of pyelonephritis (unlike simple cystitis). Blood cultures where there is fever, rigors, sepsis or immunosuppression. Also CBC, CRP, renal function and electrolytes, blood glucose, and serum lactate if unwell. Imaging is not routine but becomes mandatory if there is severe sepsis, failure to defervesce within 48–72 hours, known stones, a single kidney, diabetes, or suspected obstruction: ultrasound first (hydronephrosis, stones, abscess, post-void residual), then contrast CT, which is the investigation of choice for emphysematous pyelonephritis, renal and perinephric abscess, and pyonephrosis.

Complications and differential diagnosis. Complications: bacteraemia and septic shock, acute kidney injury, renal and perinephric abscess, emphysematous pyelonephritis (almost exclusively in diabetics — a urological emergency requiring drainage or nephrectomy), pyonephrosis, papillary necrosis, and chronic renal scarring. Differential diagnosis in an older patient: acute cholecystitis, appendicitis, diverticulitis, basal pneumonia, herpes zoster of the flank before the rash appears, renal colic, and vertebral osteomyelitis or discitis — all of which can present with fever and back or flank pain.

Current guidance stresses empirical therapy driven by the local antibiogram rather than by textbook first choices, because resistance has outrun the traditional agents: in India, fluoroquinolones and cotrimoxazole are no longer reliable empirical choices, and ICMR treatment guidelines favour intravenous amikacin, a beta-lactam/beta-lactamase inhibitor such as piperacillin-tazobactam, or a carbapenem for severe hospital-onset disease, with rapid de-escalation and switch to oral therapy once the patient has been afebrile for 48 hours. Recommended durations have shortened: about 7 days for fluoroquinolone-treated and 10–14 days for beta-lactam-treated uncomplicated pyelonephritis. Newer agents — ceftazidime-avibactam, ceftolozane-tazobactam and cefiderocol — are reserved for confirmed carbapenem-resistant organisms under stewardship control.