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There is some uncertainty and worry about the reliability of using earlobe capillary samples for assessing oxygen status in COPD patients. But is this scientific caution or just resistance to change?
The most comprehensive study of this issue is the recent meta-analysis by Zavorsky et al. The study concluded that the mean difference between arterial and earlobe capillary samples across the full range of oxygen levels is 0.3 kPa, and in the typical COPD range when assessing for oxygen support (<9.3 kPa), the difference is only 0.08 kPa. So in this clinically important range, the difference between the two methods is negligible.
Of course a comprehensive clinical programme should have the ability to use both sample types, but is your service development being held back by unfounded fears?
Ref: Arterial versus capillary blood gases: A meta-analysis. G.S. Zavorsky et al. / Respiratory Physiology & Neurobiology 155 (2007) 268–279
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I have recently completed a 50 patient paired sample audit (as yet unpublished) of cbg and abg, with regard to ltot assessment. In keeping with the majority of the current literature, including Zavorsky et al (2007)we found a large discrepency between the two techniques (optimal evidence based techniques adopted for both)Our data suggests a mean difference of 0.9kPa lower capillary pO2 than arterial.
Our audit was evaluating the impact on the outcome of the assessment which was very significant. 54% of assessments would have had a different outcome (either unnecessary LTOT initiation or repeated assessment after 6 weeks or 3 months depending on current status)
As a result, we now use Pulse oximetry, then cbg, then abg as a graduating screen in order to identify appropriate ltot pts and avoid unnecessary abg’s.
Zavorsky et al concluded:
“Sampling blood,too, from earlobe (but never the fingertip) may be appropriate as a replacement for arterial PO2, unless precision is required as the residual standard
error is 6mmHg (0.79kPa)when predicting arterial PO2 from an earlobe capillary sample” which was cited in the BTS emergency oxygen guidelines, advocating cbgs combined with pulse oximetry to assess acid base balance and oxygenation
The use of a graduated scheme is an ideal that all repiratory service providers should aspire to but a supposedly “inaccurate technique” may actually be the best (and we can’t ignore, most cost effective) option.
Firstly how inaccurate is capillary assessment? The 0.79kPa discrepancy quoted is based on measurements of “normal” individuals, at the oxygenation levels typically seen in COPD patients, Zavorsky et al report a difference of only 0.08kPa.
Secondly, regardless of the technical findings, analytical accuracy and best practice guidlines; the reality of COPD is that a vast, often unidentified proportion of patients on oxygen have never been assessed at all. Using capillary measurements in a community based service is a rapid, safe and cost effective way of assessing the needs of these patients (see case studies at http://www.axis-shielduk.com/home/near%20patient/respiratory%20disease/Case%20Studies%20and%20Articles.html). The consistent outcome of these services is that oxygen prescibing is reduced.
I completely agree that the use of capillary blood gas analysis can make huge cost savings in a population of oxygen users that have never been assessed. I have experience over the last 3 years working in an established o2 assessment team (with a caseload of 600 patients, and an average of 40 new referrals a month)and we were heavily dependant on CBG’s. The introduction of our team 5 years ago had a huge impact on the cost of o2 to the pct
However, we came across a patient who we had been monitoring on LTOT for 3 years whose pO2 on CBG was 6.7. He attended for a flight assessment which entails taking an ABG pre post test. His arterial pO2 was 8.4. The disparity was highlighted and simultaneous sampling was repeated with similar differences between the two.
We have been using best practice technique for CBG in order to improve accuracy but despite this the disparity continued. THESE PATIENTS ARE MAINLY COPD WITH BORDERLINE OR CHRONIC HYPOXIA, NOT “NORMAL INDIVIDUALS” where larger disparity has been demonstrated in the literature. However there is a considerable number of studies that have looked at hypoxaemic patients in their sample group and found similar disparity to quoted above
Our data was used to quantify the difference in assessment outcome and 24% of the sample group (n=50) would have been brought back for further assessment (unnecessarily) as they were wrongly classified as borderline, and 30% would have been started on LTOT (unnecessarily)We went on to cost this effect (in considerable detail)and calculated that a large amount of money was saved by using ABG’s (in excess of £15000 over 50 patients)
While I agree that compared to no assessment at all, cbg’s allow a vastly improved clinical assessment to take place, which has a massive cost saving implication, ABG’s can take this saving even further.
As a member of an o2 assessment service that is heavily involved in billing analysis and trending, I have seen the effect on cost that such a change in practice can have