Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
1
Is any of this happening now?
Call 911
Call emergency services if leg swelling occurs with:
New or severe difficulty breathing
Chest pain, fainting, or marked light-headedness
Coughing up blood
Blue, grey, or unusually pale lips or skin
These symptoms can occur if a clot travels to the lungs or the heart or lungs are under strain.
Other emergency signs, for anyone
Someone who is struggling to breathe, gasping, choking, or cannot get words out
Someone who has chest pain, tightness or pressure lasting more than a few minutes
Someone who has sudden weakness or drooping on one side of the face or body, slurred speech, or sudden confusion
Someone who is unconscious, or cannot be woken
Someone who is bleeding heavily and it will not stop with firm pressure
Someone who has had a seizure that will not stop, or does not wake up afterwards
Someone who has lips or skin that look pale, blue, grey or blotchy
Someone who you believe has taken an overdose or swallowed something poisonous
2
Is any of this happening?
Be seen today
Seek same-day care for sudden one-sided swelling, especially with pain, warmth, redness, or tenderness.
Get prompt care for rapidly spreading redness, fever, severe pain, a cold or pale foot, or swelling after a serious injury.
During pregnancy, seek urgent advice for sudden or marked swelling with severe headache, vision changes, upper abdominal pain, breathing trouble, or swelling of the face or hands.
These may indicate pre-eclampsia.
And for anyone, someone who:
is getting rapidly more short of breath, or has gained several pounds in a few days with swelling
3
Does any of this apply?
Book an appointment
Book a routine appointment if swelling is unexplained, lasts more than a few days, keeps returning, or worsens.
Arrange earlier follow-up for skin sores, increasing skin changes, or a known heart, kidney, liver, or vein condition.
And for anyone, someone who:
has noticed a lump, a mole that has changed, or unexplained weight loss
Every sign above is quoted from the sofidex article on Foot, leg, and ankle swelling. If you are not sure, err upward — it is always reasonable to be checked.