Investigations

• complement (C₃, C₄) ^c
• α1-antitrypsin ^c

Why?

Low complement and α1-antitrypsin reduce the chance of malignancy

Patient	Target Disorder and Reference Standard	Diagnostic Test	LR+ (95% CI)	Post-test Probability	LR- (95% CI)	Post-test Probability
pleural effusion c (pre-test probability: 47%)	malignancy (histology or follow-up)	C 3 > 300 mg/dl	1.7 (1.3 to 2.1)	59%	0.15 (0.047 to 0.46)	12%
		C 4 > 700 mg/dl	2.0 (1.6 to 2.7)	64%	0.00 (0.00 to 0.13)	0%
		α1-antitrypsin > 150 mg/dl	1.4 (1.2 to 1.7)	55%	0.00 (0.00 to 0.086)	0%