To detect inflammation, doctors often use a test that checks the erythrocyte sedimentation rate, or sed rate, in the blood. This is done by measuring the degree of rapidity with which red blood cells sink in a specimen of drawn blood. When the blood cells fall faster, the sed rate is increased and this signals the presence of inflammation. Another test looks for the presence of the C-reactive protein, because it is also a way to measure the degree of inflammation. However, because many conditions involve inflammation, neither test is considered a way to positively identify psoriatic arthritis.
A more accurate method involves the use of X-rays. X-rays can distinguish the changes in joints that are common only to psoriatic arthritis. These include changes in the distal joints found at the tips of the fingers and toes. Another change that can be seen through the use of X-ray is referred to as the "pencil-in-cup" phenomenon. The term is used to describe how the end of a bone becomes whittled down to a sharp point over time by the disease. However, since most of the changes that show up on X-rays only occur in the later stages of psoriatic arthritis, it is not considered a useful way to diagnose the disease in its earlier stages.
X-rays may also be used to look for signs that are characteristic of the disease, such as increased calcium salt deposits around the ligaments and tendons. A positive test can indicate a condition called enthesopathy, or the inflammation of the tendons, ligaments and the connecting tissues, which is common in psoriatic arthritis.
