Health condition
PNH testing after clots
A blood clot in an unusual place - abdominal veins, brain sinuses - or a clot alongside anaemia or low counts should prompt a screening test for PNH, a rare but treatable cause that is easy to miss.
Why trust this guide
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Editorial team
Written by our editorial team from the UK sources listed on each page.
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Sourced from guidance
Checked against current NHS and UK specialist society guidance at the time of review.
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Current for 2026
Reviewed on 2026-09-27 with the next review scheduled for 2027-09-27.
Key facts
The essentials at a glance.
The symptoms to recognise, when to seek help, which specialist to see, and what treatment usually involves.
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PNH testing is a simple flow cytometry blood test, no marrow needed for screening
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Test after clots in unusual sites, especially with haemolysis, low counts or dark urine
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Also indicated in aplastic anaemia, unexplained haemolysis and some MDS cases
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Finding a clone changes management, because complement inhibitors prevent further clots
Understanding the condition
What it is and what happens next.
What causes it, how it is diagnosed, and the treatment routes available in the UK.
What it is and how it feels
Most clots have conventional explanations, but a small number signal an underlying blood disorder. PNH-driven thrombosis favours unusual territories: the hepatic veins (Budd-Chiari syndrome), portal and mesenteric veins, and cerebral venous sinuses. Warning features that raise suspicion include a raised LDH, low haptoglobin, unexplained anaemia, low white cells or platelets, and episodes of dark urine.
Diagnosis and treatment routes
Screening is straightforward - a peripheral blood flow cytometry test that quantifies cells missing their protective surface proteins - and should not be forgotten once anticoagulation has started, because a positive result changes everything: complement inhibition treats the root cause rather than just the consequence. If you have had an atypical clot and PNH has never been mentioned, it is a fair question to ask. In many parts of the UK, routine NHS referrals for this kind of problem can take months, whereas an initial appointment with a private consultant is typically available within around two weeks.
Seeing a specialist
Who to see, and how quickly you can be seen.
The specialist for this condition is usually a consultant haematologist. Your GP can refer you on the NHS, or you can arrange a private consultation directly.
A quiet reminder
This guide is for information, not medical advice.
Your GP knows your history and can tell you which parts apply to you. If in doubt, see them - and seek urgent care for any of the emergency symptoms described above.
Frequently asked
Common questions about PNH testing after clots.
Quick, plain-English answers to what people ask us most.
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Why is PNH testing done after a blood clot?
PNH (paroxysmal nocturnal haemoglobinuria) is a recognised, though uncommon, cause of unusual blood clots, particularly in atypical locations such as abdominal veins, so testing is considered when a clot occurs without a clear explanation, especially alongside anaemia.
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What is PNH?
PNH is a rare acquired blood disorder where red blood cells become susceptible to destruction, which is also linked to an increased tendency to form blood clots through several related mechanisms.
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What does PNH testing involve?
The main test is flow cytometry, a specialised blood test that identifies the absence of certain protective proteins on blood cells, characteristic of PNH.
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Who should be tested for PNH after a clot?
Testing is generally considered for clots occurring in unusual locations (such as abdominal or cerebral veins), clots without an obvious cause, or when there's accompanying unexplained anaemia or dark urine.
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Is PNH dangerous if found after a clot?
Yes, it's a serious diagnosis given the ongoing risk of further clots and other complications, but effective treatments are available that significantly reduce this risk once identified.
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Can PNH be treated if identified this way?
Yes, targeted medications that block the destructive immune process can effectively control the condition and substantially reduce the future risk of clotting and anaemia.
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How long does PNH testing take?
The flow cytometry blood test itself is usually processed within a few days to a week, depending on the laboratory, providing a relatively quick answer once the sample is taken.
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PNH vs other clotting disorders — what's the difference?
Other inherited clotting disorders (thrombophilias) are typically present from birth and don't cause the anaemia or dark urine seen in PNH, which is an acquired condition affecting red blood cells directly, distinguished by flow cytometry testing.
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Should I be tested if my clot had an obvious cause?
PNH testing is generally reserved for clots without a clear explanation, so if your clot has an obvious identified cause (such as surgery or immobility), it's less likely to be recommended, though your specialist will advise based on your specific case.
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When should I see a specialist, and how quickly can I be seen privately?
See a haematologist if PNH testing has been suggested after a blood clot. Private specialist assessment is typically available within about two weeks, faster than standard NHS haematology referral pathways.