Blood in the Urine You Cannot See: What Microscopic Haematuria Means

最終更新日： 8月 27, 2026

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A urine test is part of almost every health-check package, which means a great many people are told they have red blood cells in their urine having had no symptoms and no reason to think anything was wrong. Most of the time the explanation is harmless. The purpose of the assessment is to be sure of that rather than to assume it.

A correction to the reassurance this page used to give

The earlier version of this article said, in effect, don’t worry — if it were a real problem you would see the blood with your own eyes. That is not true, and it has been removed.

Bladder cancer frequently produces bleeding that is only ever detectable under a microscope, particularly in its early stages. The reassurance was well meant and it is exactly the sentence that persuades someone to skip the follow-up appointment. The accurate version: most causes are benign, a minority are not, and which is which cannot be judged from how the urine looks.

What counts as microscopic haematuria

Three or more red blood cells per high-power field on a properly collected specimen examined under the microscope. Two points about that definition matter and were stated differently in the earlier version.

• A dipstick result is not enough. A stick that reads positive for blood must be confirmed by looking down a microscope, because several things turn a dipstick positive without any red cells being present.
• Repeat testing is not required to make the diagnosis. The earlier version described needing two consecutive tests over at least seven days; current guidance defines it on a single adequate specimen. A repeat is useful for a different purpose — excluding contamination.

Where the blood comes from

1. Contamination during collection — the commonest explanation of all, including menstruation and a poorly taken sample.
2. Kidney — stones, a mass, infection, and kidney diseases that are treated by physicians rather than surgeons.
3. Ureter — stones, or a tumour of the lining.
4. Bladder — stones, tumour, 感染.
5. Prostate — benign enlargement, 感染, cancer.
6. Urethra — stones, infection, or narrowing.
7. Vigorous exercise, which can cause transient haematuria that settles on repeat testing.

One misconception worth naming: taking a blood thinner does not explain blood in the urine. Anticoagulation may make an existing source bleed more visibly, but it does not create one, and a man on warfarin or a newer anticoagulant with haematuria needs the same assessment as anyone else. Attributing it to the tablets is a recognised route to a delayed diagnosis.

How far the investigation goes depends on your risk

The earlier version said CT with contrast is the definitive test and is what answers the question. That inverts the guideline it cited, and it has been corrected.

The central change in the 2020 American Urological Association guidance was precisely to stop sending everyone for CT. Instead, patients are placed in a low, intermediate or high risk group, and the investigation is matched to it — because CT carries a substantial radiation dose and a contrast load, and most people with a few red cells in the urine do not need one.

What determines the group:

• 年, 、および being male.
• Smoking history — the single strongest modifiable risk factor for urothelial cancer, and the one most often not asked about.
• How many red cells, and whether it has been found repeatedly.
• Any previous episode of visible blood in the urine.
• Occupational exposure to dyes, rubber, leather or industrial chemicals, sometimes decades earlier.
• Previous pelvic radiotherapy, chemotherapy with cyclophosphamide, a long-term catheter, or chronic irritative urinary symptoms.

Lower risk may reasonably mean a repeat urine test, or an ultrasound, rather than a CT and a telescope. Higher risk means the full assessment — imaging of the kidneys and ureters, and inspection of the bladder.

The part that was missing entirely: looking in the bladder

Cystoscopy did not appear anywhere in the earlier version, and no scan replaces it.

CT does not reliably show a flat tumour on the bladder lining. Carcinoma in situ can be invisible on imaging and obvious through a telescope, so for anyone in the intermediate or high risk group the assessment is imaging そして a flexible cystoscopy — a few minutes, awake, in a clinic room. A normal CT is not, by itself, an all-clear.

When it is the kidney rather than the plumbing

Some blood in the urine comes from the filtering tissue of the kidney rather than from anywhere a urologist can see. The clues are protein in the urine, abnormal-looking red cells or casts, a raised creatinine, or high blood pressure — and that combination belongs with a kidney physician, in parallel with the urological assessment rather than after it.

If everything is normal

Most assessments find nothing, and that is a genuine result rather than a failure to look hard enough. The urine is then rechecked, and follow-up can be stopped once repeat testing has come back clear.

What matters afterwards: if visible blood ever appears, that is a new event and needs assessing again regardless of how reassuring the previous work-up was.

Symptoms that need attention rather than a routine appointment

In an emergency in Thailand, call 1669.

• Blood you can see, with or without clots — this is a different and more urgent problem than microscopic haematuria, and painless visible bleeding is more concerning rather than less.
• Clots blocking the flow, or inability to pass urine.
• Fever with flank pain.
• Unexplained weight loss, or new bone pain, alongside blood in the urine.
• Severe flank pain with vomiting, which suggests a stone.

Frequently Asked Questions About Microscopic Haematuria

Is it dangerous?

Usually not — contamination and benign causes account for most of it. But it cannot be dismissed on appearance, because early bladder cancer often bleeds only microscopically. An earlier version of this article suggested a real problem would be visible to the naked eye; that was wrong and has been removed.

Do I need a CT scan?

Not necessarily. Current guidance stratifies patients into low, intermediate and high risk and matches the investigation to the group — a repeat urine test or an ultrasound may be appropriate at lower risk, reserving CT for those who need it, since it carries a significant radiation and contrast load. Age, sex, smoking history, the amount of blood, any previous visible bleeding and certain exposures determine the group.

My CT was normal. Is that the all-clear?

Not on its own for anyone at intermediate or high risk. CT does not reliably show a flat tumour on the bladder lining, so the assessment includes looking inside the bladder with a flexible telescope — a few minutes, awake, in a clinic room.

I take a blood thinner. Doesn’t that explain it?

No. Anticoagulation can make an existing source bleed more, but it does not create one, and haematuria in someone taking a blood thinner is assessed exactly as it would be otherwise. Assuming the tablets are responsible is a recognised route to a delayed diagnosis.

When can follow-up stop?

Once the assessment has found no cause and repeat urine testing has come back clear. If visible blood ever appears afterwards, that is a new event needing assessment regardless of the earlier 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 the actual urinalysis report rather than a summary, and be ready to say whether you have ever smoked and for how long — that question changes how far the assessment goes.

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.