Normal values of sacroiliac index according to gender and age

Abstract

Using quantitative sacroiliac joint scintigraphy for identifying patients with active sacroiliitis the diagnosis is relied on the abnormally high values of sacroiliac (SI) index. However, the normal range of SI index and the threshold for detecting the disease are still under discussion in the literature. The aim of the study was to determine SI indices in a sample representing normal population for establishing normal values and the optimal threshold for our laboratory. The author investigated the effect of age and gender on SI indices. SI indices in 740 nonarthritic control subjects were determined by standard method. Data were analyzed separately for males and females in different age groups (in 10-year intervals). Mean SI index derived for the control groups showed significant negative linear correlation with age and statistically significant differences between genders. The intra-individual difference between left and right SI joint was similar in each groups. The ranges of normal SI indices are different for males and females and for different age groups. For diagnosing bilateral sacroiliitis, the upper limit of 99% confidence interval is recommended as optimal cut-off value. The data between upper limits of 95% and 99% confidence intervals should be interpreted as uncertain category. The detection of unilateral sacroiliitis is possible even by the increased rate in difference between left and right SI index. The anatomic variability of sacrum should be taken into account during quantitative sacroiliac scintigraphy.