To the Editors:
In the paper by Jensen et al. 1 and the accompanying editorial by Farré and Navajas 2 the importance of quality assurance in the respiratory physiology laboratory is highlighted.
The Jensen et al. 1 study represents a tremendous and successful effort on quality assurance and quality control as part of a large clinical study. The study or probably more likely the presentation of the study by Jensen et al. 1 has a few limitations seen from a metrological point of view to which I want to draw attention. Traceability is the key stone of quality control of measurements and must be addressed in all metrological studies. Metrological traceability is a property of a measurement result relating the result to a stated metrological reference through an unbroken chain of calibrations of a measuring system or comparisons, each contributing to the stated measurement uncertainty 3. Thus, traceability refers to the completeness of the information about every step in a process chain.
In the respiratory physiology laboratory we depend on measurements with specified accuracy. Our basic measurements are volume, time, flow, pressure, concentrations, mass, length, temperature and humidity. It is within the limits of respiratory physiology laboratories to verify most of these measurements internally, but many respiratory laboratories, if any, are not able to verify gas concentration with sufficient accuracy and precision (fig. 1⇓). Therefore a clear agreement should be developed with the supplier on the methods by which conformance to purchaser's requirements will be verified 4. In the Jensen et al. 1 study this problem is obvious but most of us cannot perform an internal verification of our carbon monoxide and tracer gas and we have to rely on the supplier's verification. That's why this issue should be addressed carefully.
Concentration of gas (He) in N2. The blending tolerance is the maximum difference between the ordered concentration and the delivered concentration (in mixture). Analyses uncertainty is the maximum difference between the analyses result and the true concentration and in the example reported relative to the analysed component concentration. q.s.: quantum satis. The figure is printed with permission from AGA A/S, Linde Healthcare (Copenhagen, Denmark).
In the Jensen et al. 1 paper, the traceability of the gas concentration and volume measurements used for simulation are not stated. A reference is given to the description of the methods in a previous paper by Jensen et al. 5 in which the gas is described as a “precision gas” and the volumes are delivered by “precision syringes”. The vital importance of gas concentration accuracy and gas volume accuracy in the diffusing capacity of the lung for carbon monoxide (DL,CO) simulation study is due to the working principle of the DL,CO simulator which delivers the gas volumes and concentrations to be measured by the laboratory 1, 5. If the gas concentrations (fig. 1⇑) or volumes are not accurate within specified limits the simulations carries no validity.
To raise the questions above might seem pedantic, but as stated in the accompanying editorial the devil is in the detail 2 and traceability is not reported or overlooked to often.
The main reason for asking the questions above is not a real concern about the validity of the results, but the chance to get source information on accuracy of volume and gas concentration measurements, not easily obtainable elsewhere. The Salt Lake City laboratory (Salt Lake City, UT, USA) is highly reputed for decades of standardisation of flow measurements and has been a role model for many of us trying to improve the quality of volume and flow measurement. Therefore, we have a chance to have the difficult questions on traceability of syringe volume and gas concentration verification answered.
I hope that this type of positive industry-driven quality control will spread from industry to our clinical work now the simulator has become commercially available.
Statement of interest
None declared.
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