Kidney Stones as a Cause of Urine Colour Change

What kidney stones are and how they form

Kidney stones are crystalline aggregates that form in the urinary tract when concentrations of stone-forming compounds (calcium, oxalate, uric acid, cystine) exceed the urine's capacity to keep them dissolved. The most common type in adults is calcium oxalate (about 80 %), followed by uric acid (about 10 %), struvite (associated with infection, about 5 %), and cystine (a rare genetic stone type). The NIH NIDDK kidney stones reference covers the epidemiology and types.

The lifetime risk of forming a kidney stone is about 1 in 10 in the UK. Risk factors include male sex, age 30-60, dehydration, hot climate, high salt intake, high animal protein intake, low calcium intake (paradoxical: low dietary calcium increases oxalate absorption from gut), obesity, family history, and certain medical conditions (gout, hyperparathyroidism, inflammatory bowel disease, recurrent UTI). Some medications raise stone risk, including topiramate, indinavir, and triamterene.

Stones that remain in the kidney are usually asymptomatic. The classic stone presentation occurs when a stone moves from the kidney into the ureter (the tube from kidney to bladder), where it can lodge and cause obstruction. The combination of obstruction, peristaltic contractions of the ureter trying to push the stone past, and inflammation of the urothelial lining produces both the severe pain and the bleeding that characterise symptomatic stones.

The classic stone presentation

The classic ureteric colic presentation is severe one-sided flank pain that comes in waves, radiating from the flank around to the groin or testicle, frequently severe enough to cause nausea or vomiting. People with experience of stones often describe the pain as the worst they have ever experienced, comparable to or worse than childbirth. The Cleveland Clinic kidney stones reference describes this presentation.

Blood in urine (haematuria) accompanies the pain in most symptomatic stones. The blood may be visible (gross haematuria), producing pink, smoky, tea, or red urine, or microscopic (detected on urine dipstick). The NIH NIDDK symptoms reference lists blood in urine alongside severe pain, nausea, and vomiting as the typical symptom cluster.

Other features include increased urinary frequency and urgency (especially when the stone is near the bladder), and in some cases a change in urine appearance with cloudiness if infection coexists. Pain is typically not affected by position changes (unlike musculoskeletal pain) and people often pace or move around looking for relief.

The diagnostic workup

The NICE NG118 renal and ureteric stones guideline outlines the diagnostic pathway. Initial assessment includes a urine dipstick (looking for blood and infection), bloods (urea, creatinine, eGFR, calcium, urate), and pain assessment. The standard imaging in adults is a non-contrast low-dose CT KUB (kidneys, ureters, bladder), which has near 100 % sensitivity for stones and clarifies size, location, and any obstruction. Ultrasound is the first-line imaging in pregnancy and in children to avoid radiation.

CT also identifies stone-mimicking conditions that can present similarly: aortic aneurysm, appendicitis, diverticulitis, pyelonephritis, and gynaecological pathology. This is particularly important in older adults where stones are less likely and other diagnoses are more common.

Treatment by stone size

Below 5 mm: The NICE NG118 guideline recommends watchful waiting for spontaneous passage in most cases, supported by oral analgesia (NSAIDs are first-line where not contraindicated), oral fluids, and antiemetics for nausea. Tamsulosin (an alpha-blocker) is sometimes used to facilitate passage of distal ureteric stones. Most stones in this size range pass within four weeks.

5-10 mm: Spontaneous passage rates fall to roughly 50 % by some estimates. Watchful waiting may still be appropriate for asymptomatic or mildly symptomatic patients, but planned intervention is often discussed. The decision depends on stone location, severity of pain, presence of obstruction, and patient preference.

Above 10 mm: Surgical intervention is usually required. The main options are extracorporeal shock wave lithotripsy (ESWL, which fragments the stone using focused sound waves), ureteroscopy with laser fragmentation (a flexible scope passed up the ureter to break up the stone), and percutaneous nephrolithotomy (a small skin incision and a tract through to the kidney for larger stones). The choice depends on stone size, location, composition (some stones resist ESWL), and local urological expertise.

Obstructed and infected stone (pyonephrosis): A urological emergency. The combination of stone obstruction with infection above produces rising back-pressure, infected urine that cannot drain, and risk of sepsis. Emergency drainage by retrograde stent insertion or percutaneous nephrostomy is typically required within hours.

Prevention of recurrent stones

About half of people who have one stone will form another within 10 years. Prevention strategies depend on stone composition, which is determined when a recovered stone is sent for laboratory analysis. The general advice that applies to most stone types includes:

• -Increased fluid intake: The NIDDK and NHS guidance both recommend a target urine output of 2.5 litres per day, which usually requires drinking 2.5-3 litres of fluid per day.
• -Reduced dietary salt: The NICE NG118 guideline recommends limiting sodium intake. Salt drives calcium excretion in urine and increases stone risk.
• -Moderated animal protein: Excess animal protein increases urate and reduces urine pH, raising risk for both calcium oxalate and uric acid stones.
• -Adequate dietary calcium: Counterintuitively, low dietary calcium increases stone risk because gut calcium normally binds oxalate before absorption. Aim for normal dietary calcium intake; do not restrict.

Specific recommendations vary by stone type. Calcium oxalate stones may benefit from reducing oxalate-rich foods (spinach, rhubarb, almonds, beetroot) and adequate citrate intake (lemon water). Uric acid stones benefit from urinary alkalinisation (with potassium citrate) and allopurinol where serum urate is high. Struvite stones are infection-driven and need treatment of the underlying infection. Cystine stones are rare and need specialist management.

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