Native joint septic arthritis

Flucloxacillin IV 2g 6 hourly

and

Gentamicin* IV (dosing info here)

If true penicillin / beta-lactam allergy:

Vancomycin IV (dosing info here)

and

Gentamicin IV (dosing info here)

Obtain blood cultures, joint aspirates, and where possible bone biopsies, before starting treatment. 

A cell count and gram stain can only be performed on a joint aspirate sent in a sterile universal container.

Review gentamicin therapy at 48-72 hours as per 'Review of Gentamicin after 48-72 Hours' guidance.

*Patients with acute or chronic impairment of renal function and an eGFR <20ml/min and those with decompensated alcoholic liver disease are at increased risk of adverse events with gentamicin. IV temocillin (see here for dosing guidance) is a beta-lactam antimicrobial with a comparable breadth of gram negative cover which can replace gentamicin in these patient populations, provided they do not have a history of penicillin allergy.