A culture is a test that grows whatever bacteria are present so they can be identified. It is one of the tests that may be used, but there’s no set rule for when it should be done.
For cellulitis, the tests that may be ordered are: blood culture; complete blood count; culture of any fluid or material inside the affected area; and a biopsy, if other conditions are suspected.
May be ordered. There’s no set threshold or list of circumstances for when.
It is worth knowing that swabbing is not the main way these decisions are made. Checking for cellulitis starts with a physical exam that may reveal redness, warmth, tenderness and swelling of the skin, possible drainage if there is a buildup of pus, and swollen lymph nodes near the affected area. Pen marking — marking the edges of the redness to see whether it spreads past the border over the next several days — is a tracking method rather than a test.
For wounds generally, a wound care team examines and measures the wound, checks the blood flow in the area around it, determines why it is not healing, and creates a treatment plan. A culture is not one of those four steps.
So the honest answer is that whether to culture a wound is a clinical decision. It is a reasonable question to put to the team — what would a culture change about the treatment — and the answer should be specific.
Wound Wizard has a separate article on the tests done at a wound clinic.