What Is a Kidney Biopsy?

You've had the blood tests. You've had the urine tests, probably more than once. So why, you might reasonably ask, does your kidney specialist now want a tiny piece of the kidney itself? A kidney biopsy — doctors often call it a renal biopsy, because "renal" simply means relating to the kidneys — is a procedure in which a small sample of kidney tissue is taken and examined in a laboratory. It isn't a first step, and it isn't offered lightly. It's the step taken when the usual tests have shown that something is wrong, but not exactly what.

The short video at the top of this page, "Renal biopsy", runs for just 1 minute 20 seconds and introduces exactly that idea: tissue taken for laboratory analysis, so the type of kidney problem can be identified. The questions below follow the ones patients tend to ask next.

What can a sample show that my other tests can't?

Blood and urine tests are excellent at measuring how well the kidneys are working — tools like our eGFR calculator and urine ACR calculator work from those numbers every day. What they can't do is show the pattern of damage inside the kidney's tiny filtering structures. Under the microscope, a biopsy sample can reveal inflammation, scarring, deposits, or other changes that look quite different from one kidney condition to another, even when the blood results look similar.

That distinction matters, because different causes are handled differently. A biopsy is therefore usually considered when kidney function has fallen without a clear explanation, when there is persistent blood or protein in the urine that needs explaining, or when a transplanted kidney needs checking. Our overview of kidney failure explains the broader picture a biopsy result then fits into, and our general guide to what a biopsy is and how results are read covers how laboratories handle tissue samples of all kinds.

How is the sample actually taken?

Most kidney biopsies are percutaneous, meaning through the skin. You will usually lie face down, the skin over the kidney in your lower back is cleaned and numbed with local anaesthetic, and the clinician uses ultrasound images to find the right spot — the same kind of scanning guidance described in the anchor video. A thin biopsy needle is then passed through the skin into the kidney, often while you hold a breath for a few seconds, and a small core of tissue is collected. Sometimes more than one core is taken, so the laboratory has enough to work with.

A second video on this site, "Kidney Biopsy procedure", walks through the same idea in just over a minute, and its description puts the purpose plainly: collecting small pieces of tissue, usually through a needle, for examination with a microscope can help in forming a diagnosis and in choosing the best course. In a smaller number of cases a biopsy is done another way — for example during surgery, or through a vein — and your team will explain if one of those approaches is planned for you instead.

What happens before and straight afterwards?

Beforehand, expect blood tests, a check of your blood pressure, and a conversation about the medicines you take — particularly anything that affects how blood clots — and about allergies. Follow the instructions your own hospital sends about eating, drinking, and medicines on the day; those instructions override anything general you read, including here.

Afterwards you will be asked to rest lying down while the team keeps an eye on you, checks your blood pressure and pulse, and looks at your urine. A small amount of blood in the urine for a short time afterwards is recognised after this procedure, which is one reason for the period of observation, and the back where the needle went in may ache. You will be told who to contact once home if the pain grows, the urine becomes heavily bloodstained, or you feel unwell — take that advice seriously rather than waiting it out. In our article on polycystic kidney disease you can see another example of a kidney condition where imaging and tissue findings are weighed together before decisions are made.

When do I get an answer — and what kind of answer is it?

Not immediately, and it's worth knowing that in advance so the wait doesn't feel like bad news. The tissue has to be prepared, stained, and examined carefully, sometimes with several different laboratory techniques applied to different portions of the same sample. The report describes what the pathologist sees — which structures are involved, how much scarring is present, and what pattern the changes follow. Your kidney specialist then combines that report with your blood and urine results and your history.

Sometimes the answer is a specific diagnosis. Sometimes it narrows the field, rules out a cause that was feared, or shows how active or how longstanding a known condition is. All of those are useful answers, because each one changes what happens next — whether treatment is started, changed, or deliberately not started.

References & further reading

• MedlinePlus — Kidney biopsy: https://medlineplus.gov/ency/article/003907.htm
• National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) — Kidney Biopsy: https://www.niddk.nih.gov/health-information/diagnostic-tests/kidney-biopsy
• National Kidney Foundation — Kidney Biopsy: https://www.kidney.org/kidney-topics/kidney-biopsy

Frequently Asked Questions

Is a kidney biopsy the same as a renal biopsy?

Yes. "Renal" is the medical word relating to the kidneys, so renal biopsy and kidney biopsy are two names for the same procedure — taking a small sample of kidney tissue for laboratory examination.

Does a kidney biopsy hurt?

The skin and deeper tissue are numbed with local anaesthetic first, so the needle is usually felt more as pressure than as sharp pain. Your back may ache afterwards, and the team observing you can offer pain relief — tell them rather than putting up with it.

Will I need a biopsy if my kidney blood tests are abnormal?

Not necessarily. Many kidney problems are diagnosed and followed with blood tests, urine tests, and scans alone. A biopsy is usually considered when those tests show a problem but do not identify its cause, and the tissue answer would change how you are treated.

Can a biopsy be done on a transplanted kidney?

Yes. The same kind of sample can be taken from a transplanted kidney, which sits nearer the front of the body, to check for problems such as rejection or a return of disease. Your transplant team will explain the reason if they suggest one.

MedicalVideos.com has published trusted medical videos since 2007. The site was founded by Dr. Mohamed Ibrahim, MD, PhD — physician-scientist.

Educational only — this article is general information about kidney biopsy, not medical advice and not a diagnosis. Whether this test is right for you, and how to prepare for it, are matters for your own clinician and kidney team, who know your history.