BERLIN, Germany – Differentiating early ankylosing spondylitis (AS) from other causes of chronic back pain is difficult in the early period before radiographic sacroilitis is present. Martin Rudwaleit, MD, and colleagues in the Rheumatology Department at Charité-Campus Benjamin Franklin in Berlin and the Ankylosing Spondylitis International Federation, München, Germany, developed a method for early diagnosis based on likelihood ratios (LRs), but the calculations were intimidating. Now, the investigators have crunched the numbers and published an algorithm for bedside use.¹

Dr. Rudwaleit told CIAOMed that the typical rheumatologist can use this method to determine a patient's probability of axial spondyloarthritis (SpA) and whether additional tests are necessary. "The a priori probability of axial SpA in a patient with chronic back pain is low (5% only), and therefore, the presence of just one or two parameters (Table 1) does not suffice to make a diagnosis of early AS," he warned. "In general, you need the presence of three to four parameters in order to be confident with the diagnosis."

To use this algorithm, add the LR+ values for each parameter the patient has, and subtract the LR- values for each parameter the patient lacks. The resulting score can be read against Dr. Rudwaleit's probability curve to obtain probability of axial SpA. According to this curve