Here is info on how PA is diagnosed.
Psoriatic arthritis is a diagnosis made mainly on clinical grounds, based on a finding of psoriasis and the typical inflammatory arthritis of the spine and/or other joints. There is no laboratory test to diagnose psoriatic arthritis. Blood tests such as sedimentation rate may be elevated and merely reflect presence of inflammation in the joints and other organs of the body. Other blood tests such as rheumatoid factor are obtained to exclude rheumatoid arthritis. When one or two large joints (such a knees) are inflamed, arthrocentesis can be performed. Arthrocentesis is an office procedure whereby a sterile needle is used to withdraw (aspirate) fluid from the inflamed joints. The fluid is then analyzed for infection, gout crystals, and other inflammatory conditions. X-rays may show changes of cartilage or bone injury indicative of arthritis of the spine, sacroiliac joints, and/or joints of the hands. Typical X-ray findings include bony erosions resulting from arthritis. The blood test for the genetic marker HLA-B27, mentioned above, is often performed. This marker can be found in over 50% of patients with psoriatic arthritis who have spine inflammation.
Did you have any of these tests? For lupus there is no definitive test but often a combination of tests and symptoms are used to diagnose. In many it can take years of retesting before a definitive diagnosis is made (I have this problem). Here are different tests that can be done for lupus.
Usually your doctor will first request a complete blood count (CBC), RF and Sed rate.
Because your body’s waste is processed by the kidneys, testing a sample of urine (called a 'spot urine' test) can reveal any problems with the way your kidneys are functioning. Lupus can attack the kidneys without any warning signs, so these tests are very important. The most common urine tests look for cell casts (bits of cells that normally would be removed when your blood is filtered through your kidneys), and proteinuria (protein being spilled into your body because your kidneys are not filtering the waste properly). A collection of your urine over a 24-hour period can also give important information.
The antibodies your body makes against its own normal cells and tissues play a large role in lupus. Many of these antibodies are found in a panel, or group, of tests that are ordered at the same time. The test you will hear most about is called the ANA test. This is not a specific test for lupus, however.
Antinuclear antibodies (ANA) are antibodies that connect, or bind, to the nucleus -- the "command center" -- of the cell. This process damages, and can destroy, the cells. The ANA blood test is a sensitive test for lupus, since these antibodies are found in 97 percent of people with the disease. When three or more typical features of lupus are present -- such as involvement of the skin, joints, kidneys, lungs, heart, blood, or nervous system -- a positive ANA test will confirm a diagnosis of lupus. However, a positive ANA test result does not always mean you have lupus. The ANA can be positive in people with other illnesses, or positive in people with no illness. The ANA can also change from positive to negative, or negative to positive, in the same person. Still, lupus is usually the diagnosis when these antinuclear antibodies are found in your blood. In addition to the ANA, doctors trying to diagnose lupus often look for the following specific antibodies. Antibodies to double-stranded DNA (anti-dsDNA) are antibodies that attack the DNA -- the genetic material -- inside the cell nucleus. Anti-dsDNA antibodies are found in half of the people with lupus, but lupus can still be present even if these antibodies are not detected.
Antibodies to histone -- a protein that surrounds the DNA molecule -- are sometimes found in people with systemic lupus but are more often seen in people with drug-induced lupus. This form of lupus is caused by certain medications, and usually goes away after the medication is stopped.
Antibodies to phospholipids (aPLs) can cause narrowing of blood vessels, leading to blood clots in the legs or lungs, stroke, heart attack, or miscarriage. The most commonly measured aPLs are lupus anticoagulant, anticardiolipin antibody, and anti-beta2 glycoprotein I. Nearly 30 percent of people with lupus will test positive for antiphospholipid antibodies. Phospholipids found in lupus are also found in syphilis, and the blood test cannot always tell the difference between the two diseases. A positive result to a syphilis test does not mean that you have or ever had syphilis. Approximately 20 percent of those with lupus will have a false-positive syphilis test result.
Antibodies to Ro/SS-A and La/SS-B (Ro and La are the names of proteins in the cell nucleus) are often found in people with Sjögren’s syndrome. Anti-Ro antibodies in particular will be found in people with a form of cutaneous (skin) lupus which causes a rash that is very sun-sensitive.
Antibodies to Sm target Sm proteins in the cell nucleus. Found in 30-40 percent of people with lupus, the presence of this antibody almost always means that you have lupus.
Antibodies to RNP target ribonucleoproteins, which help to control chemical activities of the cells. Anti-RNPs are found in many autoimmune conditions and will be at very high levels in people whose symptoms combine features of several diseases, including lupus.
Complement is the name of a group of proteins that protect the body against infections. They work by strengthening the body’s immune reactions. Complement proteins are used up by the inflammation caused by lupus, which is why people with inflammation due to active lupus often have low complement levels. There are nine protein groups of complement, so complement is identified by the letter C and the numbers 1 through 9. The most common complement tests are CH50, C3, and C4. CH50 measures the overall function of complement in the blood. Low levels of C3 or C4 may indicate active lupus.
C-reactive protein (CRP) is a protein produced by the liver, and high levels of CRP in your blood may mean you have inflammation due to lupus.
The skin and kidney are the most common sites biopsied in someone who may have lupus. The results of the biopsy can show the amount of inflammation and any damage being done to the tissue.