Start with a line most people have never had drawn for them. A wound is called infected when bacteria are causing harm, not just when they are present on the skin.
That’s why a swab result on its own doesn’t settle the question. Not all bacteria seen on a swab mean infection. Doctors look mainly at clinical signs and symptoms.
Signs that may suggest infection are increasing pain, redness spreading around the wound, swelling or warmth, pus or a change in the wound fluid, bad smell, and fever or feeling unwell.
Two points come with that list, and they matter as much as the list does. Not all wounds with fluid or smell are infected, but changes over time are important. And in people with diabetes or pressure ulcers, infection may sometimes cause less obvious redness but more pain, swelling, or general illness.
What to do is simple: if you notice these changes, you should seek medical advice promptly.