The QA Pressure Measurement System: an accuracy and reliability study

Objective: The main purpose of this study was to determine the accuracy and reliability of the Queen Alexandra Pressure Measurement System (QA PMS). Furthermore, we examined whether there were significant differences in measured pressures of the buttock area during sitting between normal subjects and spinal cord injured (SCI) patients. Design: Accuracy (calibration) and reliability (test-retest) study. Setting: The spinal cord unit of Tertiary Care Centre `De Hoogstraat' in Utrecht, The Netherlands. Patients: A convenience sample of 16 SCI patients and 15 normal subjects. Main Outcome Measures: The accuracy was determined by using the Standard Error of the Mean (SEM, in mmHg). The Technical Error of Measurement (TEM, in mmHg) was calculated as measure for differences between two paired measurements. The reliability was determined by using an Intraclass Correlation Coefficient (ICC). Significant differences in measured pressures between both groups (P<0.05) were determined by using an unpaired (two sample) t-test. Results: Accuracy (calibration): mean SEM=0.30 (±0.1) mmHg, indicating a high level of accuracy. Differences between two paired measurements: mean TEM calibration=1.87 (±0.76) mmHg; mean TEM normal subjects=4.76 (±1.78) mmHg; mean TEM SCI patients=6.34 (±2.19) mmHg. Reliability: mean ICC(3,1) calibration=0.85 (95% CI=0.74–0.95); mean ICC(2,1) normal subjects=0.92 (95% CI=0.90–0.94); mean ICC(2,1) SCI patients=0.90 (95% CI=0.88–0.92). The normal subjects had significantly higher mean pressures (P=0.028) than the SCI patients (mean pressures 31.0 vs 28.5 mmHg), whilst the SCI patients had significantly higher peak-pressures (P=0.0000) than the normal subjects (mean peak-pressures: 134.1 vs 75.7 mmHg). Conclusions: The QA Pressure Measurement System has sufficient accuracy and good reliability as a measurement procedure. There are significant differences between the measured pressures of both groups: the significantly higher peak pressures of the SCI patients seem to be the most important.