4: Common respiratory investigations

Blood tests, including full blood count (FBC), provide foundational data; anaemia and polycythaemia can indicate underlying lung disease or compensatory responses to hypoxaemia. Elevated white cell, CRP, or ESR levels signify infection or inflammation. Biochemical markers like D-dimer, troponin, and calcium levels assist in identifying conditions such as pulmonary embolism, sarcoidosis, and malignancy. Immunological testing and thyroid or enzyme assessments (like ACE for sarcoidosis) help pinpoint autoimmune or infiltrative pathologies.

Sputum analysis is critical for diagnosing infections, particularly Mycobacterium tuberculosis (MTB), often requiring multiple samples or induced sputum. Pleural fluid analysis—including biochemistry, cytology, and pH—is crucial for differentiating exudate from transudate in pleural disease. Furthermore, antigen testing in urine and CSF examination are key diagnostics for severe pneumonias and CNS involvement in tuberculosis.

The Chest X-ray (CXR) remains the primary imaging tool. A systematic review of a PA view—checking for rotation, inspiration, and anatomical landmarks—helps identify consolidation, pulmonary oedema, masses, or nodules. Techniques like identifying the 'silhouette sign' or 'air bronchogram' are vital for diagnosing lobar collapse and density changes. While cost-effective and accessible, CXR requires correlation with clinical history, especially when distinguishing between malignant masses, cavitation, or miliary patterns.

Dynamic skin tests like the Mantoux (tuberculin) test and skin prick tests offer rapid, cost-effective screening for TB exposure and allergic sensitivities respectively. These tests provide immediate diagnostic clues that, when paired with imaging and blood work, support the development of comprehensive respiratory treatment plans.