Infected obstructed kidney (pyonephrosis) has a mortality of 10–40% if not drained emergently — it does not respond to antibiotics alone because the infected urine cannot drain. Temperature alone can be misleading — some patients remain apyrexial initially. AAA causes ~6,000 deaths/year in the UK; classically presents with loin/flank pain in older men and can mimic renal colic exactly — pulsatile mass on examination, or known AAA, requires immediate 999. Pregnant women are a special population: radiation risk means CT-KUB is avoided; MRI or ultrasound used, and specialist input mandatory from presentation.

The inability to find a comfortable position is a key differentiating feature of renal colic from peritonitis (where patients lie still). The classic pain radiation pattern reflects the ureteric anatomy — upper ureter pain radiates to ipsilateral testis/labia via the genitofemoral nerve. Urine pregnancy test is a mandatory first step in women — ectopic pregnancy can present identically to renal colic and delay in diagnosis is a recognised cause of maternal death (MBRRACE UK). Haematuria absent in 15% of confirmed stone cases — its absence should not cause abandonment of the diagnosis if the clinical picture fits.

Stone size is the single most important predictor of spontaneous passage — this drives the management decision of conservative vs intervention. EAU 2024 guidelines recommend medical expulsive therapy for stones 5–10mm in distal ureter. Stone composition, determined from CT Hounsfield units or stone analysis after passage, determines secondary prevention — uric acid stones are uniquely responsive to urinary alkalinisation, making the distinction clinically important. Struvite/infection stones will recur unless the underlying urease-producing organism is eradicated — antibiotics alone without stone removal will fail. Stone composition analysis should be requested for all passed stones.

The examination in renal colic is as much about excluding dangerous mimics as confirming the diagnosis. The patient who cannot keep still is a positive sign — patients with peritoneal inflammation lie motionless. Palpating for an aortic aneurysm takes 10 seconds and can be lifesaving — AAA accounts for approximately 1% of presentations with suspected renal colic, but in over-55 males with vascular risk factors, it must be actively excluded. CVA tenderness with fever raises the possibility of obstructed infected system even with normal BP/HR — these patients can deteriorate rapidly.

CT-KUB (non-contrast) is the gold standard investigation with near-perfect sensitivity and specificity, and can simultaneously identify AAA and other pathologies — it should be the investigation of choice for all non-pregnant adults. However, it involves ionising radiation (equivalent to ~1 year background radiation) — important for young recurrent stone formers who will have multiple scans over a lifetime. BAUS recommends USS as first-line for children and pregnant women. The 24-hour urine collection for metabolic workup is one of the most clinically impactful investigations in stone disease — identifying correctable metabolic abnormalities (hypercalciuria, hyperuricosuria, hypocitraturia) allows targeted prevention that can reduce recurrence by 50–80%.

The referral decision in renal colic is largely driven by whether there is obstruction + infection (the only true emergency) or whether the stone is likely to pass spontaneously (conservative management safe). EAU guidelines note that stones <5mm pass in 90% of cases without intervention — aggressive referral is not warranted. Stones >10mm have a <25% spontaneous passage rate and should be referred for urology intervention planning (ESWL, ureteroscopy, or PCNL). Recurrent stone formers (2+ episodes) benefit from systematic metabolic workup and prevention strategies, which are best coordinated through urology or specialist stone clinics.

Diclofenac is the analgesic of choice for renal colic — multiple RCTs and meta-analyses confirm NSAIDs are superior to opioids for pain control and have fewer side effects. The mechanism is prostaglandin inhibition reducing ureteric spasm and renal pelvis pressure. Medical expulsive therapy (MET) with tamsulosin is supported by NICE CKS and EAU 2024 for distal ureteric stones — it relaxes the ureteric smooth muscle via alpha-1 blockade, increasing passage rate and reducing time to passage. The evidence for stones >10mm is weaker. Importantly, dietary calcium restriction for calcium stone formers is CONTRAINDICATED — it paradoxically increases urinary oxalate and worsens stone formation. This is a common misconception that must be explicitly corrected.

Fluid intake is the single most impactful secondary prevention intervention — reducing recurrence by 50% in RCTs (Borghi et al., NEJM 2002). The 2.5L urine output target is evidence-based and should be prescribed explicitly. The calcium restriction myth is clinically important to correct actively — patients often self-restrict dairy based on a logical but incorrect assumption, which worsens their stone disease. The dietary calcium binds oxalate in the gut, reducing intestinal oxalate absorption. Straining urine to collect stones for composition analysis is free, non-invasive, and provides actionable information that guides targeted prevention — it should be recommended to every stone patient.

Recurrence rates for renal stones are 50% within 5 years without prevention — follow-up to implement prevention measures is as important as acute management. Persistent macroscopic haematuria after stone passage should not be automatically attributed to the stone — urothelial malignancy must be excluded, particularly in older patients and smokers (NICE NG12 haematuria pathway). Long-term stone disease causes progressive CKD in ~5% of patients — eGFR monitoring detects this early. The 48–72h safety-net call is particularly important because infected obstruction can develop after initial presentation appears benign — a patient who was afebrile at 9am can be septic by evening.