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To diagnose xeroderma pigmentosum (XP), a physician will perform a visual examination of the patient's skin and eyes. Along with the complete physical examination, the physician will compile a thorough medical history, including checking for a family history of XP. In most cases, clinical diagnosis of XP is based on a patient’s history of skin photosensitivity and tumors.
To further confirm an XP diagnosis, a physician may perform a skin biopsy and blood tests. The physician will then send the skin cell tissue and blood samples to a laboratory for deoxyribonucleic acid (DNA, the molecule that encodes genetic information) testing to measure the DNA repair defect following exposure to ultraviolet (UV) radiation. However, DNA testing is generally not necessary for diagnosis and usually only conducted on a research basis.
Other tests that may be performed include:
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Imaging tests (e.g., MRI, CAT scan). These imaging techniques may show the brain atrophy (deterioration) of some patients with XP with neurological symptoms (e.g., dementia).
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Audiogram test. This test measures a person’s auditory (hearing) capacity and is useful for screening for hearing loss, which is another neurological complication experienced by some patients with XP.
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Electromyography test. This test assesses nerve function and helps diagnose causes of muscular problems. An electromyography test may determine the extent of muscular damage in XP patients with ataxia (inability to coordinate muscle movements).
XP can also be diagnosed before birth with prenatal tests, such as:
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Amniocentesis. A type of prenatal test in which amniotic fluid (pregnancy fluid) is removed from the mother’s abdomen with a needle and syringe to check for abnormalities.
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Chorionic villus sampling. A prenatal testing method that uses a catheter device inserted into the vagina and cervix to obtain a sample of fetal tissue from the placenta (pregnancy sac) for laboratory analysis. Chorionic villus sampling may detect chromosomal abnormalities in the fetus. In addition, recent advances in understanding XP make it possible to test if someone is a carrier of some forms of XP by analyzing that person’s DNA. Couples with a family history of XP – especially those that have a child diagnosed with XP – may benefit from genetic counseling before considering having another child because they are at increased risk of having another baby with the hereditary disease. According to the National Institutes of Health (NIH), there is a one in four chance that any child born to the same parents of a patient with XP will also have the condition. | 
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