Low Platelets (Thrombocytopenia)
Platelets are tiny cell fragments that plug leaks in blood vessels. A mildly low count is common and often harmless, especially after a virus. Very low counts raise the risk of bleeding and need prompt attention.
Quick answer: Platelets below 150 × 10⁹/L are low. Mild drops of 100–150 are common after viral infections, with some medicines, alcohol or in pregnancy, and often need just a repeat test. Below 50, bleeding after injury becomes more likely. Below 20 needs same-day medical review; below 10 is an emergency.
What the test measures
The platelet count is part of a full blood count (FBC). Platelets are made in the bone marrow, live about a week, and clump together with clotting proteins to stop bleeding. A low count is called thrombocytopenia.
Typical adult ranges
- Normal: 150–400 × 10⁹/L (150,000–400,000 per microlitre in US units)
- Mildly low: 100–149
- Moderately low: 50–99
- Severely low: under 50
A false low is common. Platelets sometimes clump in the blood tube (pseudothrombocytopenia), so the machine undercounts them. A blood film or a repeat sample in a different tube (citrate) sorts this out.
What a low result usually means
Common causes:
- A recent viral infection, including glandular fever, flu and COVID-19. Counts usually recover within weeks.
- Medicines: heparin, some antibiotics, valproate, quinine and others.
- Alcohol and liver disease: alcohol slows platelet production, and an enlarged spleen from liver disease holds on to platelets.
- Immune thrombocytopenia (ITP): the immune system destroys platelets. One of the commonest causes of an isolated low count.
- Pregnancy: gestational thrombocytopenia affects up to 1 in 10 pregnancies, is usually mild and settles after birth.
- Low B12 or folate. See low B12.
Less common but important:
- Leukaemia, lymphoma, myelodysplasia or other bone marrow problems, usually with other abnormal counts.
- Heparin-induced thrombocytopenia (HIT): a fall of more than half, 5–10 days after starting heparin. Paradoxically it causes clots.
- Thrombotic thrombocytopenic purpura (TTP) and haemolytic uraemic syndrome (HUS): low platelets with broken red cells, kidney problems, fever or confusion.
- HELLP syndrome in late pregnancy, and sepsis with abnormal clotting (DIC).
- Rare inherited platelet disorders.
Symptoms
- Often none, especially above 50 × 10⁹/L.
- Easy bruising, and tiny red or purple dots on the skin that don’t fade when pressed (petechiae), often on the lower legs.
- Nosebleeds or bleeding gums, heavy periods, or bleeding that takes longer to stop after a cut.
- Blood in urine or bowel motions.
What happens next
- Repeat the FBC with a blood film to rule out clumping and look at the other cells.
- Review medicines, alcohol and recent illnesses.
- Other tests may include B12 and folate, liver tests, HIV and hepatitis C, and a pregnancy test.
- A mild, stable drop with normal other counts and no bleeding is usually rechecked over a few weeks.
- Haematology referral for a new moderate or severe drop without a clear cause, a falling count, or other abnormal blood cells.
- Until you know more, avoid aspirin and anti-inflammatories such as ibuprofen unless a doctor says otherwise. Paracetamol is usually fine.
See a doctor the same day if your platelets are under 20 × 10⁹/L (20,000 per microlitre): hospital admission is often advised when this is a new finding. Under 10 × 10⁹/L is an emergency. Get urgent help (000 in Australia, or your local emergency number) at any count for bleeding that won’t stop, vomiting blood, black or bloody bowel motions, a sudden severe headache, or low platelets with confusion, fever or reduced urine. Tell a doctor straight away if your count falls by half within 5–10 days of starting heparin, or if you’re pregnant with headache, upper tummy pain or high blood pressure.
Practise reading results. Play Panelysis, the daily blood-test puzzle: a fictional patient, a real-looking panel, three guesses. More in medical games.
Common questions
What platelet count is dangerous?
Bleeding risk rises as the count falls, but not in a straight line. Above 50 × 10⁹/L most people have no symptoms; 20–50 can cause easy bruising and longer bleeding after injury; under 20 needs same-day review; and under 10 carries a high risk of spontaneous bleeding and is treated as an emergency.
Is a platelet count of 120 bad?
It’s mildly low and often harmless. Common reasons include a recent virus, alcohol, some medicines or pregnancy, and some people simply run a little low. Your GP will usually repeat it with a blood film and look at your other counts.
Can a virus lower platelets?
Yes. Many viral infections, including glandular fever, flu and COVID-19, can lower platelets for a few weeks. A repeat test after you’ve recovered usually shows the count coming back up.
Can low platelets be a sign of leukaemia?
Occasionally, but leukaemia usually affects the other blood counts too, and often causes symptoms such as fevers, weight loss, tiredness or swollen glands. A mildly low platelet count with otherwise normal results is far more likely to have a harmless cause.
What should I avoid if my platelets are low?
Avoid aspirin and anti-inflammatories like ibuprofen unless your doctor says otherwise, as they make platelets work less well. Go easy on alcohol. If your count is very low, avoid contact sports and tell your dentist or surgeon before any procedure.
Are low platelets in pregnancy normal?
A mild fall is common: gestational thrombocytopenia affects up to 1 in 10 pregnancies and settles after birth. Your midwife or doctor will still check it, because conditions like pre-eclampsia, HELLP and ITP can also lower platelets.
Evidence and more information
- Pathology Tests Explained (RCPA): Platelet count (range and causes)
- Gauer and Whitaker, American Family Physician 2022: Thrombocytopenia, evaluation and management (severity, red flags, pseudothrombocytopenia)
- American Society of Hematology 2019: ITP guideline pocket guide (admission under 20 × 10⁹/L, treatment under 30)
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