NIHR Signal Potential alternative to painful blood tests in people with flare-ups of COPD
Published on 27 September 2016
It may be possible to use venous blood and pulse oximeters to initially assess the severity of a flare up of chronic obstructive pulmonary disease (COPD). Blood gas levels were similar in people when testing blood from either arteries (the recommended test) or veins. Oxygen saturation using pulse oximeters – which attach to the end of the finger – were also reasonably accurate compared to testing arterial blood.
Flare-ups of COPD – where the person’s condition suddenly deteriorates – are the second most common cause of emergency hospital admissions in the UK. It is important to assess how serious a flare-up is by testing blood for gases such as carbon dioxide and oxygen. Taking an arterial blood sample is painful and technically challenging. So this study looked at whether analysing blood from a vein and pulse oximeter measures could be an alternative, less invasive and simpler test.
Although promising, we still need to understand and define cut-offs in the various measurements, to indicate when arterial blood samples are required to confirm or clarify the findings of alternative blood gas analyses.
- Diagnostics, Respiratory disorders, Acute and general medicine
Why was this study needed?
COPD causes difficulty breathing that can severely affect daily living. In the UK, 900,000 people are diagnosed with COPD, but as many as two million are undiagnosed.
COPD symptoms can be managed through medication and self-care. However, COPD flare ups (exacerbations) are the second most common cause of emergency hospital admissions and 50% of people with a severe exacerbation will die within four years.
During an exacerbation, oxygen levels are reduced, but carbon dioxide can increase, making the blood more acidic. In order to give the right amount of oxygen, the levels of oxygen, carbon dioxide and the acidity of the blood (pH) are measured using a blood sample, usually taken from an artery.
This NIHR funded cohort study set out to investigate whether there were situations where analysing blood gases from venous samples or the use of pulse oximetry could be an alternative.
What did this study do?
This study analysed 234 people’s blood gases while they were hospitalised for a COPD exacerbation in a single UK hospital. Samples were taken from arteries and veins, and pulse oximetry – a sensor that clips onto the end of your finger, to compare the accuracy of these different measurements.
Arterial and venous blood samples were taken using standard methods, and analysed using the same blood gas analyser based on the ward. Blood gas samples have to be analysed quickly to ensure accuracy and taking the two samples did introduce some delay, but this did not affect the results overall.
What did it find?
- Blood pH was similar between arterial (7.40) and venous (7.37) blood samples, (mean difference 0.03, 95% confidence interval [CI] 0.02 to 0.04).
- There was a greater difference in average carbon dioxide levels in arterial (6.89kPa) and venous (7.63kPa) blood samples (mean difference ‑0.75, 95% CI ‑0.89 to ‑0.61).
- Average hydrogen carbonate levels were similar between arterial (29.7mEq/L) and venous (29.7 mEq/L) blood samples, (mean difference -0.04, 95% CI -0.22 to 0.15).
- Pulse oximeter readings were accurate when the oxygen saturation was at 80% or more. Mean pulse oximeter readings were 91.0% and arterial blood samples were 91.2%, (mean difference -0.17, 95% CI -0.89 to 0.56).
- Arterial blood gas samples were more painful to obtain and required more attempts, 90.2% of venous samples were obtained first time, whereas only 69.2% of arterial blood gas sample were successful first time.
What does current guidance say on this issue?
The NICE 2010 guideline recommends that arterial blood gas measurements are taken in all people referred to hospital with an exacerbation of COPD. Where facilities to measure arterial blood gases are not available, NICE recommends that oxygen saturation is measured using a pulse oximeter.
2008 guidance from the British Thoracic Society acknowledged the potential use of venous blood gas but did not make recommendations because it was not widely used. An update of these guidelines is due for publication in 2016.
What are the implications?
Overall there was good agreement with arterial and venous samples for blood acidity and hydrogen carbonate, and between arterial samples and pulse oximeter measurements. There was more of a difference in carbon dioxide levels, which could be significant around certain cut-off points.
This study suggests that it may be possible to use venous blood gas results to initially assess COPD flare-ups if pulse oximeters indicate that the oxygen saturations are at least 80% and the pH is less than 7.35. The authors stress that if this were to be adopted as a strategy, that arterial samples should be performed if there is “a risk or actual clinical deterioration”. This could reduce the need for painful and technically challenging arterial blood samples, which may require multiple attempts by experienced staff.
Before any change to practice, there is a need to define cut-offs for arterial blood sampling and to agree an evidence-based algorithm for practice.
Citation and Funding
McKeever TM, Hearson G, Housley G, et al. Using venous blood gas analysis in the assessment of COPD exacerbations: a prospective cohort study. Thorax. 2016;71(3):210-5.
This project was funded by the National Institute for Health Research (NIHR) research for patient benefit programme (grant number PB-PG-0211-24049).
NICE. Chronic obstructive pulmonary disease in over 16s: diagnosis and management. CG101. London: National Institute for Health and Care Excellence; 2010.
NHS Choices. Chronic obstructive pulmonary disease. London: Department of Health; 2014.
British Lung Foundation. Chronic obstructive pulmonary disease (COPD) statistics. London: British Lung Foundation; 2016.
I agree that a venous sample with satisfactory pH and pCO2 level can effectively exclude the possibility of hypercapnia (abnormally raised carbon dioxide). Arterialised capillary blood gas samples (from earlobes or finger prick) have similar advantages over arterial samples for many patients but are vulnerable to errors of technique and require a high level of staff training. It is likely that venous blood gases will be used more widely in the future but there are two important hurdles to overcome. Firstly, front-line staff will need to be trained to interpret venous blood gas samples. Secondly, it can be difficult to identify in case records if a particular blood gas sample was arterial, arterialised or venous and this could lead to mis-interpretation of the results.
Dr Ronan O’Driscoll, Consultant Physician, Respiratory Medicine, Salford Royal Foundation NHS Trust