The first step in diagnosing CSU is a thorough physical examination and medical history. Doctors look for the characteristic hives and swelling that define CSU. They'll ask about how long symptoms have been present, how often they occur, and any potential triggers. To be classified as CSU, hives must persist for at least 6 weeks without an obvious cause.

While there's no specific blood test for CSU, doctors often order several tests to rule out other conditions. These typically include a complete blood count, which can detect signs of infection or blood disorders. Tests for inflammation markers like C-reactive protein or erythrocyte sedimentation rate may also be done. Some doctors check thyroid function, as CSU is associated with thyroid disorders in some patients.

In most cases, a skin biopsy isn't necessary to diagnose CSU. However, if the doctor suspects a different condition like urticarial vasculitis, they may take a small sample of skin for examination under a microscope. This can help distinguish CSU from other skin disorders that cause similar symptoms.

In some cases, doctors may use specialized tests to learn more about a patient's CSU. The autologous serum skin test, which involves injecting a small amount of the patient's own serum under the skin, can indicate if autoantibodies might be involved. However, this test isn't routinely used as it doesn't change treatment approaches. Researchers are exploring other potential biomarkers for CSU, but these remain experimental.

While diagnosing CSU can be challenging, a combination of clinical evaluation and targeted testing can help identify this condition accurately.