Sensitivity & specificity of tests for Porphyria

Sensitivity is a feature of many of the diagnostic tests for porphyrias, especially when they are done at or near the time of symptoms. Tests that are sensitive for diagnosis of active porphyrias are almost always abnormal when symptoms of Porphyria are present. But the tests vary in specificity, meaning that some of the tests (those with lower specificity) are abnormal in other diseases.

In choosing a test to screen for a disease that is not only uncommon but also causes symptoms that mimic more common diseases, it is obviously important to choose a test that is both sensitive and specific. Fortunately, some tests for Porphyria have both of these features. With such tests, it should always be possible to determine if symptoms might be due to one of the porphyrias.

Table 1. Diseases due to deficiencies of specific enzymes of the heme biosynthetic pathway

The enzymes and their intermediates (substrates and products) are shown in sequence. Some intermediates are porphyrinogens (reduced porphyrins); these are excreted and measured mostly as oxidized porphyrins (click on any of the yellow-highlighted enzyme names to determine if the substances should be measured in blood, urine or feces). A deficiency of the first enzyme, ALAS, causes a type of anemia rather than a Porphyria. (Uroporphyrinogen I synthase is an obsolete term for PBGD and is not shown in the Table.)

Click to enlarge chart

Porphyria should be suspected quite frequently, because the symptoms that suggest the diagnosis are common. If a diagnosis of Porphyria is not made promptly, serious consequences to the patient may follow. But more often than not, laboratory testing will show that the patient does not have Porphyria, because other diseases with symptoms that mimic Porphyria are much more common. An effective approach to laboratory testing is one that does not often miss the diagnosis and also does not often incorrectly suggest that Porphyria is present.