Testing

About lab tests

Approach to laboratory testing—Screening followed by confirmation
Many tests are offered for the diagnosis of one of the porphyrias. One might think that this is an advantage in diagnosing porphyrias. However, it may be difficult to choose the proper tests and interpret the results, resulting in delay in diagnosis and even misdiagnosis of porphyrias. Tests may be overused, leading to unnecessary expense.

In the past, it was customary to obtain many or all available tests when porphyria was suspected and then examine patterns of any abnormal results. Abnormal results might be compared with tables showing patterns of abnormalities to be expected in the various porphyrias. This approach did not adequately consider the sensitivity and specificity of the various tests or how abnormal the results should be for making a diagnosis of porphyria.

A better and less costly approach is to rely on a few screening tests that are both sensitive and specific. If a well-chosen screening test is normal, the possibility of porphyria as a cause of symptoms is effectively ruled out. If the test is positive, further testing is needed to establish which type of porphyria the patient has. The screening tests may be regarded as "first-line" tests, and the tests that follow (when a screening test is positive) as "second-line tests." Consultation with a porphyria expert may be needed to choose and interpret the tests that should follow a positive screening test.

An approach that prioritizes laboratory testing in this manner has many advantages. It is cost effective, because porphyria can be ruled in or out in many patients with nonspecific symptoms with only one or a few tests. As already noted, porphyria is ruled out much more commonly than it is ruled in, and therefore one or a few negative screening tests can be effective and also keep medical costs down. Relying on a few screening tests that have a high degree of specificity also avoids misdiagnosis of porphyrias.