• 
• 

Three Part Question

In [a patient with an acute exacerbation of COPD] is [a capillary blood sample as good as an arterial blood sample] at measuring [PaO2, PaCO2 and pH]?

Clinical Scenario

A 60 year old man presents to the Emergency Department with an acute exacerbation of COPD. Analysis of his blood gases is required. You wonder whether a capillary blood sample will be as accurate as an arterial blood sample.

Search Strategy

Medline 1966-11/00 using OVID interface.
{[Capillar$.mp AND (exp blood gas analysis OR blood gas$.mp)] AND [(exp arteries OR arter$.mp) AND (exp blood gas analysis OR blood gas$.mp)]} LIMIT to human AND english.

Search Outcome

280 papers found of which 276 were irrelevant or of insufficient quality. The remaining 4 papers are shown in the table.

Relevant Paper(s)

Author, date and country	Patient group	Study type (level of evidence)	Outcomes	Key results	Study Weaknesses
Langlands JH and Wallace WF, 1965, UK.	14 patients under investigation for pulmonary disease and 2 normal patients. Arterial blood vs capillary blood from ear.	Diagnostic test	PO2	Mean difference 0.62 mm Hg (SD 4.1) NS	Small numbers No power calculation Do not know if any patients were suffering from COPD
			PCO2	Mean difference 1.05 mm Hg (SD 1.6) NS
			pH	Mean difference 0.006 (SD 0.01) NS
Begin R et al, 1975, USA.	45 patients in acute respiratory distress without circulatory shock. 15 were below age 16. Arterial vs capillary blood from finger.	Diagnostic test	PO2	Mean difference 2.1 mm Hg (SD 4.4) r=0.97	Small numbers No power calculation Do not know if any of these patients were suffering from COPD
			PCO2	Mean difference 1.4 mm Hg (SD 3.2) r=0.98
			pH	Mean difference 0.006 (SD 0.016) r=0.98
Pitkin AP et al, 1994, UK.	40 patients with chronic lung disease and a variety of arterial blood gas tensions. 29 had COPD and bronchiectasis. Arterial blood vs capillary blood from ear.	Diagnostic test	PO2	Mean difference 0.17 kPa (CI -1.09 to +0.75)	Small numbers No power claculation Patients suffered from a variety of underlying illnesses.
			PCO2	Mean difference 0.21 kPa (CI -0.24 to +0.67)
			pH	Mean difference 0.007 (CI -0.008 to +0.022)
Dar K et al, 1995, UK.	55 patients requiring measurment of blood gases. 22 had exacerbations of COPD. Arterial vs capillary blood from ear.	Diagnostic test	PO2	Mean difference 0.09 kPa (SD 0.59)	Small numbers No power calculation
			PCO2	Mean difference 0.01 kPa (SD 0.3)
			pH	Mean difference 0.007 (SD 0.02)

Comment(s)

Different studies have given slightly different results. There have been no statistically significant differences identified. Moreover the differences that have been seen are clinically insignificant as well. Further research in patients with COPD would be useful.

Clinical Bottom Line

Properly taken capillary blood samples accurately reflect arterial blood gas measures of PO2, PCO2 and pH.

References

1. Langlands JH, Wallace WF. Small blood samples from ear-lobe puncture. The Lancet 1965;2:315-7.
2. Begin R, Racine T, Roy JC. Value of capillary blood gas analysis in the management of acute respiratory distress. American Review of Respiratory Disease 1975;112:879-881.
3. Pitkin AD, Roberts CM, Wedzicha JA. Arterialised earlobe blood gas analysis: an underused technique. Thorax 1994;49:364-6.
4. Dar K, Williams T, Aitken R, et al. Arterial versus capillary sampling for analysing blood gas pressures. British Medical Journal 1995;310:24-5.