There are two people asking this question. One has a clot already, often diagnosed abroad, and wants to know whether they can get home. The other is worried that a flight will give them one.
This article is mainly for the first. The short answer is that most people with a deep vein thrombosis can fly once their treatment is established, and that a pulmonary embolism calls for more caution and a closer look at breathing.
What the danger actually is
A deep vein thrombosis, or DVT, is a blood clot in one of the deep veins, usually in the leg. The clot itself causes pain and swelling. The danger is that part of it breaks away and travels to the lungs, where it becomes a pulmonary embolism, or PE.
The risk is highest at the start, before anticoagulant treatment has taken effect. Anticoagulants do not dissolve the clot directly. They stop it growing and stop new clots forming while the body breaks the existing one down, and that is why the start of treatment matters more than the date the clot was found.
Flying adds to clot risk in ways that are mostly about the seat rather than the sky. Hours of sitting still, knees bent, with less fluid taken than usual, slow the blood in the legs. The cabin altitude plays a smaller part. For a healthy traveller the added risk is low, and research summarised by the World Health Organization commonly cites flights of more than about four hours as the point where it begins to rise.
How long to wait: DVT and pulmonary embolism
After a DVT. Published aeromedical guidance commonly allows travel once a patient is established on anticoagulant treatment and their symptoms are stable or improving, rather than setting a long fixed interval. For the newer tablet anticoagulants, which work within hours, that can mean a short wait. For warfarin, which takes days to reach the right level, it means waiting until blood tests show the dose is right, and planning further checks around the trip.
After a PE. The lungs have been affected, so the question becomes how well they have recovered. Commonly applied guidance asks that the patient is anticoagulated, stable, not breathless at rest and not needing oxygen on the ground before a commercial flight is considered. The interval is usually longer than after a DVT and is set by the treating team rather than a table, because the size of the embolism and its effect on the heart vary so widely.
If oxygen is still needed, the cabin will make that need greater rather than leave it the same. Supplemental oxygen can be arranged with most airlines, but it needs lead time and its own paperwork. Where the picture is borderline, the same hypoxic challenge test described in can you fly with a chest infection can answer the question on the ground.
Airlines may ask for airline medical clearance after a recent clot, particularly a PE. A letter from the treating doctor giving the diagnosis, the date, the treatment and a statement that you are stable makes that straightforward.
Lowering the risk on the flight
None of this replaces advice from the team treating you, but these are the measures commonly recommended, including in NHS advice for travellers.
Keep taking your anticoagulant exactly as prescribed, and carry it in hand luggage with enough to cover delays. If you are crossing time zones, ask in advance how to time your doses rather than working it out at the gate.
Move. Walk the aisle when it is safe to, and flex your ankles and calves while seated. An aisle seat makes this much easier, and it is worth asking for one when booking.
Drink water, and go easy on alcohol. Avoid sleeping tablets, which keep you still for hours at a time.
Ask whether compression stockings are right for you. Properly fitted graduated stockings are commonly advised for travellers at higher risk, but the fit matters and so does the state of the affected leg.
Know the warning signs of a PE: sudden breathlessness, chest pain that is worse on breathing in, or coughing up blood. They need urgent help wherever you are, including on board.
Finally, tell your travel insurer about the clot. An undeclared condition is one of the commonest reasons a later claim is questioned, as hospitalised abroad: who pays explains.
If you are stuck abroad, or managing a case
Clots diagnosed abroad often follow something else: an injury, an operation or a leg in plaster. The broken leg case is a common one, and can you fly with a broken leg covers how the cast and the clot interact.
Most patients with a treated DVT travel home on an ordinary seat with a sensible plan. A patient recovering from a PE who is still breathless, or who needs oxygen, may need a medical escort and oxygen arranged in advance, and occasionally a stretcher. As with every case, the clinical picture should choose the transport, which is the principle set out in what good repatriation actually looks like.
If you are arranging this for a family member or managing it for an insurer, medical repatriation and patient transport explains how the move is put together. With a clot, the details that matter most are the date treatment started, which anticoagulant was used, and whether the lungs were involved.
