Discussion
BLOOD
In a randomised cross-over study of 118 adults aged 45–74 years, we found evidence of differences in measurement of blood pressure, grip strength and lung function when assessed using different devices. For blood pressure, the newer Omron HEM-907 measured higher than the older Omron 705-CP with wide limits of agreement. For grip strength, the two electronic dynamometers recorded measurements on average 4-5kg higher than either the hydraulic or the spring-gauge dynamometer, but there were only small mean differences when comparing the two electronic dynamometers or the hydraulic and spring-gauge dynamometers. However, limits of agreement were wide for all comparisons. For lung function, the ndd Easy on-PC measures of FVC were an average of 0.47 litres higher than those for the Micro Medical, but there was no difference between measures of FEVWe are aware of only a few studies that have compared combinations of dynamometers previously. For example, King [We have not identified a comparison of Micro Medical or other turbine spirometers with the ndd Easy on-PC spirometer. However, in a study of 35 volunteers, the Micro Medical turbine spirometer, used in our study, gave lower readings compared with the Vitalograph Micro pneumotachograph spirometer [For lung function, the accuracy of measurement relies primarily on optimal coaching: maximally deep breath, a rapid blast and appropriate encouragement as well as a full seal around the mouthpiece and correct body posture [In considering the potential clinical significance of the differences between devices, we have referred to published normative or predicted values of blood pressure, grip strength and lung function [Maintaining consistency in the make and model of device used in studies reduces the likelihood of measurement differences, but is not always realistic given that equipment becomes obsolete and new technology can improve measurement, for example through automation (as is the case with the Omron 907), the transition from analogue to digital (as is the case with the transition from the Jamar hydraulic to Jamar Plus+ devices) or the introduction of visual encouragement and specific feedback (as provided by the ndd Easy on-PC). An important implication of our findings is that it would be advisable for researchers, therefore, to include simple experiments to assess machine comparability when a new device is introduced into a study. Conducting external comparison studies, such as ours, would also help interpretation for both within-study and between-study comparisons. In addition, the differences between devices need to be considered in the context of reliability of measurements for each device being compared. Our analysis showed good reliability of measurements, particularly for the dynamometers and spirometers, suggesting the differences observed are important. The ATS/ERS quality control for lung function ensures excellent reliability, but does result in exclusion of those who cannot meet the criteria.A key strength of this study design was that it used the same standardised measurement protocols for all devices, which is important, as for all three functional measures, the type of device is only one of several factors which can affect measurements unless these other factors are kept constant as in our study. Blood pressure is affected by multiple factors [In conclusion, this randomised cross-over study showed measurement differences between devices commonly used to assess blood pressure, grip strength and lung function which researchers should be aware of when carrying out comparative research between studies and within studies over time.