Abnormal Kidney Function: When to Get a Kidney MRI

Abnormal kidney function tests — raised creatinine, low eGFR, or abnormal urea — are found in roughly 6–7% of UK adults. Most of the time, mild abnormalities are stable and manageable. But certain patterns in your kidney blood tests should prompt imaging to check the kidneys' structure, blood supply, and surrounding anatomy. Here's exactly when a kidney MRI is warranted.

Understanding Your Kidney Blood Results

eGFR (Estimated Glomerular Filtration Rate)

eGFR estimates how well your kidneys filter waste. It's the single most useful kidney function marker.

• Normal: >90 mL/min/1.73m² (Stage 1 — normal kidney function)
• Mildly reduced: 60–89 (Stage 2 — usually no symptoms, monitor)
• Moderately reduced: 45–59 (Stage 3a — mild-to-moderate CKD)
• Moderately-severely reduced: 30–44 (Stage 3b — needs specialist input)
• Severely reduced: 15–29 (Stage 4 — significant CKD)
• Kidney failure: <15 (Stage 5 — dialysis or transplant may be needed)

Creatinine

• Normal range: Men: 60–110 µmol/L. Women: 45–90 µmol/L.
• What raises it: Kidney dysfunction (main concern), but also dehydration, high-protein diet, intense exercise, and certain medications (ACE inhibitors, NSAIDs).

Urea

• Normal range: 2.5–7.8 mmol/L
• Less specific than creatinine — affected by diet, hydration, and GI bleeding.

When to Get a Kidney MRI

Kidney imaging (ultrasound first, MRI if needed) is indicated when:

Imaging Triggers

• eGFR <60 on two tests 3+ months apart — Confirmed chronic kidney disease. Imaging checks kidney size, structure, and for obstruction.
• Rapid decline in eGFR — A drop of >5 per year or >15 over 3 years suggests an active process that needs investigation.
• Haematuria (blood in urine) — Even microscopic haematuria with abnormal kidney function needs imaging to rule out kidney masses, stones, or vascular abnormalities.
• Proteinuria (protein in urine) — High protein in urine alongside reduced eGFR suggests glomerular disease — imaging helps assess kidney size and structure.
• Recurrent kidney stones — MRI can detect stones and assess for structural abnormalities that predispose to stone formation.
• Unexplained kidney size difference — If ultrasound shows one kidney is significantly smaller than the other, MRI can investigate the blood supply (renal artery stenosis).
• Kidney mass on ultrasound — MRI provides detailed characterisation of kidney masses — distinguishing simple cysts (harmless) from solid masses (potentially malignant).

Kidney MRI vs Ultrasound vs CT

Important note: If your eGFR is below 30, discuss gadolinium contrast with your doctor before booking an MRI. There's a rare risk of nephrogenic systemic fibrosis (NSF) with certain gadolinium agents in severe kidney disease. Modern agents (gadobutrol, gadoterate) have a much better safety profile, but it's worth discussing.

What a Kidney MRI Shows

• Kidney size and shape: Small kidneys suggest chronic damage; large kidneys can indicate polycystic kidney disease or infiltration.
• Masses and cysts: MRI can classify cysts using the Bosniak system (I–IV), determining whether they're simple (benign) or complex (potentially malignant).
• Renal artery stenosis: Narrowing of the artery supplying the kidney — a treatable cause of kidney dysfunction and high blood pressure.
• Obstruction: Hydronephrosis (swelling due to blocked urine flow) — caused by stones, strictures, or external compression.
• Cortical scarring: Evidence of previous infection (pyelonephritis) or reflux disease.

The Clinical Pathway

1. Abnormal kidney bloods found → Repeat in 2–4 weeks to confirm.
2. Confirmed abnormality → Check urine (dipstick for blood and protein). Kidney ultrasound as first-line imaging.
3. Ultrasound normal, stable bloods → Monitor with regular blood tests (3–6 monthly).
4. Ultrasound shows a mass or abnormality → Kidney MRI for detailed characterisation.
5. Rapid decline in eGFR or haematuria → May go directly to MRI for urgent assessment.

Related Guides

• When Blood Tests Suggest You Need a Scan
• Elevated Liver Enzymes → Liver MRI
• MRI vs CT vs Ultrasound
• MRI Scan Cost by Body Part
• Blood Test Then Scan: Complete Pathway