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 now if you develop:
Trouble breathing, wheezing, or throat tightness
Swelling of the tongue, lips, face, or throat
Fainting, collapse, confusion, or extreme weakness
Severe, widespread muscle spasms that affect breathing or walking
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 urgent medical care for intense or rapidly worsening pain, repeated muscle cramps, persistent vomiting, chest symptoms, or a bite involving a young child.
Eye pain, redness, blurred vision, light sensitivity, or suspected urticating hairs in the eye also need prompt assessment.
And for anyone, someone who:
has a wound that is hot, spreading red, or leaking pus
3
Does any of this apply?
Book an appointment
Routine follow-up is appropriate when redness, swelling, or tenderness does not steadily improve, or when signs of infection develop.
Seek review if the wound becomes hotter or more painful, drains pus, or is accompanied by fever.
The article also says
Contact a poison centre or medical service if the species may be an Old World tarantula, symptoms extend beyond the bite area, or you are uncertain what bit you.
If available, provide a photograph or the spider's scientific name without risking another exposure.
Every sign above is quoted from the sofidex article on Tarantula spider bite. If you are not sure, err upward — it is always reasonable to be checked.