Table 1:

Useful basic clinical tests

Clinical test	Rationale
Routine tests
Complete blood cell count	Abnormal blood cell counts can suggest infection or cancer*
Serum LDH, protein	To apply Light’s criteria
Pleural LDH, protein	To apply Light’s criteria
Pleural pH, glucose	To identify complicated parapneumonic effusions
Pleural fluid culture	To identify infectious agents
Pleural fluid cytology	To diagnose malignant disease
Contrast chest CT for exudates	To identify mediastinal/pleural/lung lesions

Note: CT = computed tomography, LDH = lactate dehydrogenase. Light’s criteria: An effusion with any of the following characteristics is classified as an exudate: pleural:serum ratio > 0.5, pleural:serum LDH ratio > 0.6 or pleural LDH > 2/3 of the upper limit of normal for the serum. An effusion with none of these characteristics is classified as a transudate.⁶

^*For a more detailed discussion on the value of cellular counts and differentials, readers should consult the guideline published by the British Thoracic Society.¹