Mantoux Test Reading: How Many mm Is Positive and What It Means
Answer first: a Mantoux test is read by measuring the hard, raised bump (induration) across your forearm 48 to 72 hours after the injection, not the redness. For most people in Pakistan and India, 10 mm or more counts as positive, and 5 mm or more counts for people with HIV or close TB contacts. A positive result means your body has met the TB germ at some point. It does not, on its own, mean you have active TB.
Before you read your arm
- What’s measured: the firm swelling you can feel, in millimetres, across the arm. Redness is ignored.
- When: 48 to 72 hours after the injection, by a trained reader.
- Common cut-offs: 5 mm, 10 mm or 15 mm, depending on your risk. Your doctor applies the right one.
- What positive means: TB infection, past or present. Active TB needs symptoms, an X-ray and often a sputum test.
- Other names: tuberculin skin test (TST), PPD test.
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The Mantoux is old. It still confuses almost everyone. Parents panic over a big red patch. Adults wave off a small hard lump, go home, and never come back for the reading at all. Here’s how it actually works, as part of our Lab Reports Decoded series.
How does the Mantoux test work?
A nurse injects a tiny amount of purified protein from the TB germ (tuberculin, or PPD) just under the top layer of skin on the inner forearm, which raises a small pale bubble that fades within minutes. Then you wait.
If your immune system has met TB before, its memory cells recognise the protein and rush to the spot over the next two to three days. That crowd of cells makes the skin firm and raised. If your body has never met TB, nothing much happens. Flat skin. Think of it as knocking on a door and seeing who answers, because a person who has lived in that house before will come running, while a house that has never had a visitor stays silent however long you knock. The test checks memory. It can’t tell whether the germ is asleep in your body or causing active disease right now.
How to read a Mantoux test: induration, not redness
People get this wrong. Constantly.
- Induration is the hard, raised area you can feel with your fingertips. This is what gets measured.
- Erythema is the redness around it. It can be much bigger. It doesn’t count.
- The reader looks at the arm 48 to 72 hours after the injection, in good light.
- They gently run a fingertip or a ballpoint pen from outside toward the centre until it stops at the edge of the firm bump, on both sides.
- They measure across the forearm (side to side, not along the arm), in millimetres, with a flexible ruler.
- The number is written as “induration X mm”. No bump at all is written as 0 mm.
If more than 72 hours pass, a clear positive can often still be read, but a negative isn’t reliable and the test may be repeated. Self-reading at home tends to go wrong, so go back to the clinic.
How many mm is positive?
Cut-offs depend on how likely you are to have met TB and how weak your defences might be. These are widely used guides; your doctor may apply local programme rules.
| Induration | Usually counted as positive for |
|---|---|
| 5 mm or more | People with HIV, close contacts of someone with active TB, people on steroids or other immune-lowering medicines, anyone whose X-ray already looks like old TB |
| 10 mm or more | Most people in high-TB countries such as Pakistan and India, children under 5, health workers, people with diabetes or kidney failure |
| 15 mm or more | Anyone, even with no known risk |
Blisters, or a very large reaction, are sometimes noted as “strongly positive”. They look alarming, and parents understandably rush back to the clinic, but they still don’t prove active disease.
Does the BCG vaccine make the test positive?
It can. Most people in South Asia got the BCG vaccine at birth, the small round scar on the upper arm. BCG uses a weakened cousin of the TB germ, so the immune system may remember it and give a small reaction. That effect usually fades over the years, so a big reaction in an adult who was vaccinated as a newborn several decades ago is less likely to be explained by BCG alone and more likely to reflect real contact with TB somewhere along the way. Because BCG and TB exposure are both so common here, doctors sometimes prefer a blood test called IGRA (for example the QuantiFERON test), which isn’t affected by BCG. It costs more and isn’t always available.
A positive Mantoux never means “start TB medicine now” by itself, and a negative one doesn’t clear someone who is coughing, losing weight or sweating at night. Babies, people with HIV, severe malnutrition or a recent measles infection can test falsely negative. If there’s a cough lasting over two weeks, coughing blood, fever with weight loss, or a child who is not gaining weight, see a doctor or your TB programme centre. Coughing blood or breathlessness needs emergency care today. Nobody should take TB medicine, or preventive medicine, unless a doctor has prescribed and planned it.
What happens after a positive result
The doctor asks about cough, fever, night sweats and weight loss, checks the chest with a stethoscope, asks who at home or work has had TB, and usually orders a chest X-ray to look for any shadow in the lungs. If there’s a cough, a sputum test such as GeneXpert for a lingering cough may follow. Then one of two things:
- Active TB: symptoms, X-ray changes or a positive sputum test. This needs the full course of treatment through the TB programme, free in Pakistan and India.
- TB infection only (latent TB): the germ is asleep and not spreading. Some people, especially young children in contact with TB and people with HIV, are offered preventive medicine that a doctor plans. Others are simply watched.
A positive Mantoux answers “have you met TB?”, never “are you sick with TB?”
How to prepare, and caring for the arm
No fasting. None. Tell the nurse about recent live vaccines (measles, MMR), because the test is usually done the same day as them or about four weeks later. After the injection, don’t scratch, rub, bandage or put creams on the spot. Plain water on it while bathing is fine. Itching? Normal. Go back on time.
Mantoux test questions, answered
How many mm is a positive Mantoux test?
For most people in Pakistan and India, 10 mm or more of hard swelling is positive. For people with HIV, close TB contacts or weak immunity, 5 mm is enough. Above 15 mm counts as positive in anyone.
Is redness counted in a Mantoux test?
No. Only the firm raised bump (induration) is measured. A large red area with a small or no bump is still read by the bump.
When should a Mantoux test be read?
Between 48 and 72 hours after the injection. Reading too early or too late can give a misleading answer, especially a false negative.
Does a positive Mantoux test mean I have TB?
No. It means your body has met the TB germ at some point. Only a doctor, with symptoms, a chest X-ray and sometimes sputum tests, can say whether TB is active.
Can BCG cause a positive Mantoux test?
Yes, usually a small reaction, and it tends to fade with age. A large reaction in an adult is less likely to be BCG alone, and the doctor may use an IGRA blood test to check.
What is a normal Mantoux test result in a child?
Under 10 mm is usually read as negative in a healthy child here, and 0 mm is common. Children who live with someone with TB are judged at 5 mm, so tell the doctor about any contact.
Can a Mantoux test be falsely negative?
Yes. Very young babies, malnourished children, people with HIV, recent measles, some medicines and severe TB itself can all blunt the reaction. A negative test doesn’t overrule symptoms.
Why is my Mantoux arm itchy and swollen?
Itching and swelling are the immune reaction the test looks for. Don’t scratch or apply creams before the reading. If it blisters or ulcerates, show the nurse.
Mantoux test vs IGRA: which is better?
IGRA (a blood test) isn’t affected by BCG and needs only one visit, but it costs more and isn’t everywhere. Both check TB infection, not active disease. The doctor picks based on age, cost and availability.
Can I take a bath after a Mantoux test?
Yes. Water is fine. Just don’t rub the spot, cover it with a plaster or put creams on it.
Can a Mantoux test be done in pregnancy?
Yes. It’s considered safe in pregnancy. Tell the doctor you’re pregnant so any follow-up X-ray is done with shielding.