For a patient with a suspected deep skin infection that is not responding to initial treatment, the necessary investigations include microbiological sampling such as biopsy or aspirate of the affected tissue or abscess to identify causative organisms and guide antibiotic therapy.
Imaging techniques such as X-ray, CT, or MRI should be considered to assess the extent of the infection, especially if there is suspicion of deeper involvement like bone or joint infection.
Routine laboratory tests including blood cultures may be indicated if systemic infection or dissemination is suspected.
Histological examination of biopsy samples can also be useful to exclude other causes such as tuberculosis or malignancy.
Microbiological culture and sensitivity testing are essential to tailor antibiotic treatment, especially if initial empirical therapy has failed.