Around half of all venous clots are linked to a hospital stay, and most of those are preventable. NICE's guideline on preventing them (NG89) requires every adult admitted to hospital to be assessed for clot risk and bleeding risk on the day they arrive, and again 24 hours later or whenever their condition changes. This chapter is what that assessment should produce, written so you can check it did, followed by the everyday risks that are in your own hands.
1. In hospital: the assessment every admission must have
The assessment scores the factors that raise clot risk (age, cancer, obesity, immobility, a previous clot, surgery, infection, hormones, and others) against the factors that raise bleeding risk (active bleeding, a recent brain or spinal operation, a low platelet count, an anticoagulant already prescribed). The decision is then one of three:
- Injections: low-molecular-weight heparin once a day (or fondaparinux), for anyone whose clot risk outweighs their bleeding risk. For a medical admission this means most people who will be in bed or a chair for much of the day.
- Sleeves: intermittent pneumatic compression, inflatable cuffs on the calves or feet that squeeze rhythmically, where injections are contraindicated by bleeding risk, and routinely after a stroke, where injections are not given.
- Nothing, for people who are mobile and low risk, with the assessment repeated if that changes.
Anti-embolism stockings are used less than they were: they do not work after a stroke and NG89 dropped them for medical patients; they remain an option around surgery where injections cannot be used. A previous DVT or PE is one of the highest-weighted items on the form, so say so at every admission.
2. After surgery: how long the injections continue
The clot risk after an operation does not end at discharge; for the biggest operations it peaks in the second and third weeks. NG89 sets durations by operation, and the most common are:
| Operation | NICE (NG89) | Note |
|---|---|---|
| Hip replacement | Heparin injections for 10 days then aspirin for a further 28; or heparin for 28 days with stockings; or rivaroxaban tablets | Five weeks in total |
| Knee replacement | Aspirin for 14 days; or heparin for 14 days with stockings; or rivaroxaban | Two weeks |
| Hip fracture | Heparin for a month, or fondaparinux, starting after admission and continuing after surgery | |
| Abdominal or pelvic surgery for cancer | Heparin for 28 days after the operation | Four weeks, at home, self-injected |
| Other major abdominal, thoracic, gynaecological or urological surgery | Heparin for at least 7 days, with sleeves in hospital | |
| Day surgery, minor procedures, lower-limb casts | Assessed individually; heparin considered for a leg in a cast if risk factors are present | |
| Caesarean section | Assessed by the RCOG score; heparin for 10 days is common, six weeks with higher risk | RCOG, not NG89 |
If you are discharged after one of these without either the injections to take home or an explanation of why not, ask. The self-injection is taught on the ward; district nurses can help. The American guidelines reach similar durations by a different route and are more willing to use aspirin or a DOAC alone after joint replacement.
3. Medical admissions, and the weeks after
Heart failure, pneumonia, a severe infection, an inflammatory flare, a stroke or any illness that keeps someone in bed all carry a clot risk comparable to surgery, and the injections are given for the length of the stay. Continuing them after discharge from a medical admission is not routine: trials found more bleeding than benefit in most, though a short course is reasonable for someone going home still immobile with other risk factors.
4. At home: what is in your hands
| Factor | What to do |
|---|---|
| Immobility at home, after an illness, injury or operation | Move every hour you are awake; calf exercises in a chair; a cast is a reason to ask about injections |
| Sitting still for four hours or more — a flight, a car, a coach, a train, a desk | The risk roughly doubles and is the same for every mode of travel. Aisle seat or a stop every one to two hours, walk hourly, calf exercises, water, alcohol modest, no sedatives; below-knee stockings for the journey if you have had a clot or are otherwise at higher risk; a single heparin injection before the highest-risk journeys. Not aspirin. The full before-during-after table is in the PE lifestyle chapter |
| The combined pill, patch, ring and oral HRT | Raise the risk two- to four-fold; contraindicated after a clot; transdermal HRT is the lower-risk form. Contraception and HRT after a clot are in the same chapter |
| Pregnancy and the six weeks after | The highest-risk period of a woman's life for a clot; tell the midwife about any previous clot or family history; injections are given to those at risk |
| Obesity | Roughly doubles the risk; weight loss lowers it |
| Smoking | A modest risk factor for clots and a large one for everything else |
| A family history | A first-degree relative with a clot roughly doubles your risk; it changes the threshold for prophylaxis, not the need for testing (see the thrombophilia section) |
| Dehydration | Widely believed, weakly evidenced; drink normally |
| Varicose veins | A risk factor for superficial thrombosis; treatment of troublesome varicose veins reduces it |
5. If you have had a clot before
Everything above applies with the threshold lowered. Tell every admitting team and every surgeon; expect injections for any operation and most admissions; ask about a short course of injections for a long-haul flight or a period of immobility; and if you are still on an anticoagulant, the surgical team needs a plan for when to stop and restart it, made by the prescriber, not the dentist or the anaesthetist on the day.
6. Where UK and US guidance differ
- The universal admission assessment is a UK national requirement with an audit behind it; US hospitals use risk scores (Padua, Caprini) without a single mandated process.
- After joint replacement, US guidance accepts aspirin alone more readily; NG89 accepts it after a heparin lead-in for hips and alone for knees.
- Extended prophylaxis after medical admission: neither recommends it routinely.
- Stockings: both have retreated from them for medical patients.