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

BACKGROUND

Plantar 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.

METHODS

Bench testing placed the PressureStat under loads throughout the range reported for PressureStat (0-15.0 kg/cm(2)). 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.

FINDINGS: In 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.

INTERPRETATION: Pressure 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.