The validity and reliability of PressureStat™ for measuring plantar foot pressures in patients with rheumatoid arthritis

BackgroundPlantar pressure measurements are a useful screening tool to assess foot function and identify patients at risk of developing ulceration. The validity and reliability of PressureStat™ for measuring plantar pressures in patients with rheumatoid arthritis has not been addressed.MethodsBench testing placed the PressureStat™ under loads throughout the range reported for PressureStat™ (0–15.0 kg/cm2). The outputs were digitised to measure pixel density in comparison with the PressureStat™ calibration card. In clinical testing, simultaneous barefoot plantar pressure measurements were recorded from 10 rheumatoid arthritis patients using the PressureStat™ and EMED-ST systems. Reliability was examined by four independent examiners using kappa statistics. The absolute peak pressure values in each mask region were determined from the EMED-ST platform by masking in the same manner using a real size plantar pressure printout.FindingsIn all test conditions, the pixel density recorded from the PressureStat™ outputs exceeded the range recorded on the calibration card, with the largest difference in the lower ranges. In clinical testing, correcting the PressureStat™ scores for ‘background noise’ (not suggested by manufacturer) inverted and improved these comparisons. Inter-observer kappa values between the first and second readings ranged from 0.26 to 0.59, with some evidence of a learning effect. Intra-observer kappa values were 0.57, 0.51, 0.33 and 0.43 for observers 1–4 respectively.InterpretationPressure data obtained with the PressureStat™ system in bench testing and patients with RA is inaccurate, particularly in the lower ranges. The grading of PressureStat™ readings taken from rheumatoid arthritis patients found fair to moderate intra and inter-observer agreement suggesting that interpretation is also imprecise.