Investigations

• biochemistry:
  • protein ^c
  • LDH ^{b c}
  • glucose ^{b c}
  • pH ^b

  Note:

• Biochemical tests can help distinguish exudates and transudates, but remember up to 1% of patients with malignancy have transudative pleural effusions. ^c

Why?

A raised protein or LDH level, or a low glucose makes an exudate more likely

Patient	Target Disorder and Reference Standard	Diagnostic Test	LR+ (95% CI)	Post-test Probability	LR- (95% CI)	Post-test Probability
pleural effusion b (pre-test probability: 74%)	exudate (biopsy, imaging, follow-up)	LDH > 0.45 upper limit of normal	4.8	93%	0.15	30%
		protein > 2.9 g/dl	5.4	94%	0.10	22%
pleural effusion c (pre-test probability: 85%)	exudate ( biopsy, cytology )	Light's criteria: any abnormal LDH, LDH ratio or protein ratio	4.3 (2.5 to 7.4)	96%	0.036 (0.017 to 0.076)	17%
pleural effusion c (pre-test probability: 49%)	exudate (final diagnosis)	glucose 6.0 mmol/l or less	5.0 (2.8 to 8.8)	83%	0.31 (0.21 to 0.47)	23%

A low glucose increases the risk of a complicated parapneumonic effusion

Patient	Target Disorder and Reference Standard	Diagnostic Test	LR+ (95% CI)	Post-test Probability
parapneumonic effusion b (pre-test probability: 40%)	complicated effusion (frank pus or requiring drainage)	glucose < 2.0 mmol/l	16 (5.2 to 50)	91%
		2.0 to 5.0 mmol/l	0.92 (0.47 to 1.8)	38%
		< 5.0 mmol/l	0.28 (0.16 to 0.47)	15%

A raised LDH increases the risk of a complicated parapneumonic effusion

Patient	Target Disorder and Reference Standard	Diagnostic Test	LR+ (95% CI)	Post-test Probability
parapneumonic effusion b (pre-test probability: 40%)	complicated effusion (frank pus or requiring drainage)	LDH > 1600 U/ml	8.1 (3.3 to 20)	81%
		600 to 1600 U/ml	1.0 (0.53 to 2.0)	36%
		< 600 U/ml	0.29 (0.15 to 0.55)	14%

A pH > 7.3 reduces the risk of a complicated parapneumonic effusion

Patient	Target Disorder and Reference Standard	Diagnostic Test	LR+ (95% CI)	Post-test Probability
parapneumonic effusion b (pre-test probability: 40%)	complicated effusion (frank pus or requiring drainage)	pH < 7.2	9.9 (5.7 to 17)	87%
		7.2 to 7.3	1.2 (0.63 to 2.4)	45%
		>7.3	0.086 (0.042 to 0.18)	5%