Residual Urine: What the Number Means, and What It Cannot Tell You

最終更新日： 8月 28, 2026

• ワードプレス
• LinkedIn
• メール

Patients ask this constantly: why does the doctor want a scan of my bladder after I have emptied it? The answer is that emptying and passing urine are not the same thing. A bladder can produce a respectable stream and still leave a substantial volume behind, and that stagnant urine is what causes infections, stones and, at the extreme, pressure on the kidneys.

• 泌尿器センター バンコク病院 タイ オンライン予約 02-310-3009 bhquro@bdms.co.th
• サミティジ・シラチャ病院 チョンブリ 088-022-1445

How it is measured

It used to mean passing a catheter after you had finished, which was accurate and unpleasant. Ultrasound has replaced it: the bladder is scanned, you go to the toilet, and it is scanned again. The result is the volume left behind, in millilitres, without anything being inserted.

Two things affect the accuracy, and both are worth knowing because they are the reason a surprising result should not be acted on immediately.

• The second scan must follow quickly — within about ten minutes. The kidneys keep producing urine, so a delayed scan measures new urine as well as leftover urine and overstates the result.
• It depends on who is scanning. The volume is calculated from measured bladder dimensions, so technique matters.

Practically: come with a comfortably full bladder and do not empty it in the waiting room, or the test cannot be done and you will be sent away to drink and come back.

A single reading should not decide anything

This was absent from the earlier version and is the most important addition to the page.

Residual volume varies enormously in the same person from one void to the next. How much you had drunk, how long you had held on, how relaxed you were, whether the clinic toilet felt private — all of it moves the number. A man can measure 250 mL on one visit and 40 mL on the next with nothing having changed.

So a single high reading is a reason to repeat the measurement, not a reason to start treatment. And a result is only meaningful if a reasonable volume was actually passed: emptying 60 mL and leaving 60 mL behind tells you nothing about the bladder, because the bladder was never properly full to begin with — the same principle that governs flow testing.

What the numbers mean

• Under about 50 mL — emptying well.
• Over about 200 mL — emptying poorly, and worth acting on.
• In between — the range the earlier version left unaddressed, and where most results actually fall. It is interpreted alongside symptoms, flow rate, kidney function and whether infections keep occurring, rather than treated as a verdict in itself.
• In children, the threshold is lower and paediatric assessment differs.

The earlier version presented a separate normal range of 50 to 100 mL for older people. That has been softened: residual volumes do tend to rise with age, but this is a tendency rather than an established age-specific normal, and a raised residual in an older man is not automatically to be accepted as age. These figures are guides for interpretation, not diagnostic cut-offs. Persistently over about 100 mL is associated with more urinary infections, which is a practical reason to take repeated moderate elevations seriously.

What the number cannot tell you

Also absent from the earlier version, and the reason the measurement is a starting point rather than an answer.

A high residual volume does not say why the bladder is not emptying, and there are two quite different explanations that look identical on the scan:

• The outlet is blocked — an enlarged prostate or a urethral stricture.
• The bladder muscle is weak — from long-standing obstruction, diabetes affecting the nerves, neurological disease, or age.

The distinction decides the treatment, and getting it wrong matters: an operation on the prostate of a man whose real problem is a weak bladder does not fix his emptying. Where it is unclear, pressure-flow studies settle it. Medication is also worth reviewing before anything else — anticholinergics, older antihistamines, decongestants and opioids all impair emptying, as set out under causes of retention.

Symptoms that need attention the same day

In an emergency in Thailand, call 1669.

• Complete inability to pass urine with a painful full lower abdomen.
• 発熱または寒気と震え with difficulty passing urine.
• Constant dribbling with a full, uncomfortable abdomen — an overflowing bladder rather than an improvement.
• A very large residual with rising kidney blood tests, which needs draining rather than observing.
• New leg weakness or numbness, or loss of bowel control, alongside difficulty emptying — an emergency measured in hours.

Frequently Asked Questions About Residual Urine

What is residual urine and why is it measured?

It is the volume left in the bladder after you have finished passing urine. It is measured because a good stream does not prove good emptying, and urine left behind causes infections, stones and, at the extreme, pressure on the kidneys.

My result was high. Does that mean I need treatment?

Not on one reading. Residual volume varies substantially between voids depending on fluid intake, how long you held on and how relaxed you were, so a high result is a reason to repeat the measurement rather than to start treatment. The result also only means something if a reasonable volume was passed.

What counts as normal?

Under about 50 mL is good emptying and over about 200 mL is poor. Most results fall in between and are interpreted alongside symptoms, flow rate, kidney function and any history of infection. Persistently over about 100 mL is associated with more urinary infections. These figures guide interpretation rather than serving as diagnostic cut-offs.

Does a high residual mean my prostate needs an operation?

Not necessarily. A blocked outlet and a weak bladder muscle produce identical results on the scan, and operating on the prostate of a man whose bladder is the real problem does not improve his emptying. Pressure-flow studies distinguish them, and medication that impairs emptying should be reviewed first.

How should I prepare?

Arrive with a comfortably full bladder and do not empty it in the waiting room. The second scan must follow within about ten minutes of voiding, since the kidneys keep producing urine and a delayed scan overstates the result.

相談の手続きをする

Dr. Soarawee Weerasopone sees patients at バンコク病院本部 そしてチョンブリー県のサミティベート・シーラチャ病院にて 088-022-1445. Bring any previous residual measurements and flow tests with their dates, and a full list of your medicines — several common drugs impair bladder emptying and are worth excluding before anything else is considered.

バンコク病院の遠隔医療は、来院が困難な患者様（海外からの患者様を含みます）を対象にご利用いただけます。泌尿器科宛てに事前にメールでご手配ください。 bhquro@bdms.co.th. サミティベート・シラチャは対面診療のみです。料金に関するお問い合わせは当ウェブサイトではなく、病院にお問い合わせください。.

免責事項 この記事の内容は、バンコク病院本社（Bangkok Hospital Headquarters）の認定泌尿器科医であるソラウィー・ウィーラソポン医師（Dr. Soarawee Weerasopone）が執筆および監修したものであり、教育目的のみを意図しています。これは医学的なアドバイス、診断、または個別の処方ではなく、個人的なメッセージングチャンネルやソーシャルメディアを通じてアドバイス、診断、処方が行われることはありません。ソラウィー医師は公開のソーシャルメディアアカウントを運営していません。彼の名前で個人的な相談を提供しているアカウントはすべて詐欺です。タイで緊急事態が発生した場合は、以下にお電話ください。 1669.