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
Get urgent or emergency help for:
Severe difficulty breathing, blue or grey lips, or struggling to speak
New confusion, unusual drowsiness, collapse, or difficulty waking
A rapidly spreading rash with fever
Severe weakness with cold, mottled, or unusually pale skin
Signs of severe dehydration, such as very little urine
Symptoms of a serious allergic reaction during treatment
These symptoms can occur with sepsis or another serious problem.
Use your local emergency number rather than waiting for a routine appointment.
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 prompt medical advice for a new fever, worsening cough, painful skin infection, persistent vomiting or diarrhoea, or symptoms that are not improving as expected.
3
Does any of this apply?
Book an appointment
Arrange routine medical advice if you or your child has repeated infections, infections that keep returning, persistent thrush, unexplained weight loss, poor growth, long-lasting diarrhoea, or a strong family history of severe infection.
And for anyone, someone who:
has noticed a lump, a mole that has changed, or unexplained weight loss
The article also says
If you already have an immunodeficiency, follow your agreed plan for fever and new infection symptoms.
Every sign above is quoted from the sofidex article on Immunodeficiency diseases. If you are not sure, err upward — it is always reasonable to be checked.