Investigations

• arterial blood gases ^c , using local anaesthetic ^a , if SaO₂ ≤ 92% or signs of hypercapnia 

Why?

• PEFR or FEV1 alone cannot safely exclude hypercapnia or hypoxia. ^c

Peak expiratory flow rate cannot safely rule out hypercapnia or hypoxia.

Patient	Target Disorder and Reference Standard	Diagnostic Test	LR+ (95% CI)	Post-test Probability	LR- (95% CI)	Post-test Probability
acute asthma c (pre-test probability: 18%)	hypoxia (pO 2 < 8 kPa on arterial blood gas)	PEFR ≤ 25% predicted	1.3 (0.92 to 1.8)	22%	0.59 (0.24 to 1.4)	11%
acute asthma (pre-test probability: 4.5%)	hypercapnia (pCO 2 > 6 kPa on arterial blood gas)	PEFR ≤25% predicted	1.2 (0.64 to 2.1)	5%	0.71 (0.13 to 4.0)	3%

• Arterial blood gas sampling is more painful than venous sampling - local anaesthetic before sampling reduces pain. Moreover injecting local anaesthetic is not clearly more painful than saline ^a
• The procedure takes around 40 seconds longer to perform ^a