What initial investigations should I consider for a patient presenting with a fever of unknown origin (PUO) lasting more than one week?

For a patient presenting with a fever of unknown origin (FUO) lasting more than one week, initial investigations should include a thorough clinical assessment to identify any symptoms or signs of serious illness or specific diseases. Initial laboratory tests typically include a full blood count, blood cultures, C-reactive protein, and urine testing for urinary tract infection. Chest X-ray should be considered if respiratory signs are present. In infants younger than 3 months, additional investigations such as lumbar puncture are indicated if the infant appears unwell or has abnormal white blood cell counts. Stool culture may be performed if diarrhoea is present. Observation in hospital with or without further investigations may be necessary to differentiate serious from non-serious illness. These investigations help to identify common causes of prolonged fever such as bacterial infections, urinary tract infections, pneumonia, meningitis, or other inflammatory conditions. Immediate treatment decisions, including parenteral antibiotics, depend on clinical severity and investigation results. NICE NG143