CT scans appear to help in diagnosis of gout

According to research presented at the Nov 2011, American College of Rheumatology Annual Scientific Meeting in Chicago, a type of computed tomography scanning (commonly called CT scans) appears to help in the diagnosis of gout by detecting the urate crystals that are often a symptom of the disease.

Gout is a painful and potentially disabling form of arthritis that has been recognized since ancient times. Initial symptoms of gout usually consist of intense episodes of painful swelling in single joints, most often in the feet (especially the big toe). Gout occurs when excess uric acid (a normal waste product) accumulates in the body, and needle‐like uric acid crystals deposit in the joints. Large deposits of uric acid crystals, called tophi, may also deposit both in joints and in the tissues around joints. This may happen because either uric acid production increases or because the kidneys are unable to remove uric acid from the body adequately.

A type of CT scan called dual energy CT scan (commonly called DECT) is highly accurate in spotting kidney stones, and this technology has recently been modified to detect monosodium urate crystals. Researchers from the Mayo Clinic in Rochester, Minn. recently assessed the accuracy of using DECT scanning to diagnose gout as a complementary or alternative way for physicians to make a diagnosis of the disease (in addition to or instead of withdrawing and examining synovial fluid from a person's joint).

To assess this, the researchers formed two study groups: the first group included 40 people who had been diagnosed with gout based on the presence of uric acid crystals in their joint fluid. The second group was comprised of 40 people with other types of joint disease and a negative joint fluid analysis.

Researchers grouped participants with gout based on what joint was affected and how long they had experienced symptoms (six weeks or less vs. over six weeks). All participants with gout had dual energy CT scans of their affected joints, and a radiologist reviewed the results of each scan - noting whether urate crystals were detected. These CT scan results were compared to the results of the joint fluid analysis.

"By grouping our study participants according to symptom duration and location of the affected joint, we wanted to make sure that our study population represented a real-world spectrum of disease," explains lead investigator Tim Bongartz, MD, consultant in rheumatology at Mayo Clinic, Rochester, Minn. "We didn't want to make it too easy for the scan to pick up the correct diagnosis by only including patients with long standing, tophaceous disease and large uric acid deposits. We wanted to really challenge the new method by including patients who were only a few days into their first flare of gout."

The second part of the study further explored the scan's usefulness as tool for gout diagnosis. Researchers formed a third group of 30 participants who were suspected to have gout, but couldn't receive a confirmation from traditional testing. These participants underwent the same CT scanning as the others. If the radiologist confirmed the presence of urate crystals, an ultrasound was performed to guide the removal of synovial fluid in the joint with a needle and syringe to confirm diagnosis.

"Our group thought that merely demonstrating the dual energy CT scan's ability to pick up uric acid deposits would not be very useful for its possible introduction into clinical practice," explains Dr. Bongartz. "After demonstrating the accuracy of a new test, the real question becomes whether it will affect clinical decisions and contribute to improved patient care. This is especially true when the new test is far more expensive than the established standard test we already have at hand."

The study found that dual energy CT scanning was very accurate in correctly identifying patients with gout. Among 40 patients with gout, only three were missed with the CT scan. Conversely, the scan indicated presence of uric acid in two out of 40 patients who had a joint fluid analysis that was negative for uric acid crystals. Among all three cases that were missed belonged to the group of patients who only had joint pain for a few days and who had never experienced a gout flare before.

"Our results did reveal that dual energy CT scanning is very accurate in correctly identifying patients with gout," says Dr. Bongartz. "But, as good as the overall results are, they also do indicate that there are subgroups of patients where physicians have to be more cautious in interpreting results. For example, the CT scan failed to reveal the correct diagnosis in 30 percent of cases with very acute gout. The diagnostic yield part of our study reveal the strength of this new way of diagnosing gout: in identifying cases where an appropriate specimen for analysis cannot be obtained or joint fluid analysis is negative for the presence of uric acid crystals. For this patient group, dual anergy CT scanning does provide a very useful method to quickly get to the correct diagnosis and direct the patient towards the appropriate therapy."