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PORPHYRIAS DIAGNOSIS |
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Depending on the array of symptoms
presented, the possibility of porphyria may not immediately come to
mind. In the absence of a family history of porphyria, some symptoms
of porphyria, such as abdominal pain and vomiting, may be attributed
to other disorders. Neurological symptoms, including confusion and
hallucinations, can lead to an initial suspicion of psychiatric
disease rather than a physical disorder. Diagnosis may be aided in
cases in which these symptoms appear in combination with neuropathy,
sensitivity to sunlight, or other factors. Certain symptoms, such as
urine the color of port wine, are hallmark signs of porphyria. A common initial test measures protoporphyrins in the urine. However, if skin sensitivity to light is a symptom, a blood plasma test is indicated. If these tests reveal abnormal levels of protoporphyrins, further tests are done to measure heme precursor levels in the stool and in red blood cells. The presence and estimated quantity of porphyrin and protoporphyrins are easily detected in biological samples using spectrofluorometric testing. This procedure involves the use of a laboratory instrument called a spectrofluorometer that directs light of a specific strength at a fluid sample. Certain molecules in the sample--such as heme precursors--absorb the light energy and fluoresce. When molecules fluoresce, they emit light at a different strength than the absorbed light. The fluorescence can be detected and quantified by the spectrofluorometer. Not all molecules fluoresce, but among those that do, the intensity and quality of the fluorescence is an identifying characteristic. Whether heme precursors occur in the blood, urine, or stool gives some indication of the type of porphyria, but more detailed biochemical testing is required to determine their exact identity. Making this determination yields a strong indicator of which enzyme in the heme biosynthesis pathway is defective; which, in turn, allows a diagnosis of the particular type of porphyria. Biochemical tests rely on the color, chemical properties, and other unique features of each heme precursor. For example, a screening test for acute intermittent porphyria (AIP) is the Watson-Schwartz test. In this test, a special dye is added to a urine sample. If one of two heme precursors--porphobilinogen or urobilinogen--is present, the sample turns pink or red. Further testing is necessary to determine whether the precursor is porphobilinogen or urobilinogen--only porphobilinogen is indicative of AIP. Other biochemical tests rely on the fact that heme precursors become less water soluble (able to be dissolved in water) as they progress further through the heme biosynthesis pathway. For example, to determine whether the Watson-Schwartz urine test is positive for porphobilinogen or urobilinogen, a measure of chloroform is added to the test tube. Chloroform is a water-insoluble substance, and even after vigorous mixing, the water and chloroform separate into two distinct layers. Whether the chloroform layer or the water layer becomes pink indicates which heme precursor is present. Porphobilinogen tends to be water soluble, and urobilinogen is slightly water insoluble. Since like mixes with like, porphobilinogen mixes more readily in the water than chloroform; therefore, if the water layer is pink, an AIP diagnosis is probable. As a final test, measuring specific enzymes and their activities may be done for some types of porphyrias; however, such tests are not done as a screening method. Certain enzymes, such as porphobilinogen deaminase (the defective enzyme in AIP), can be easily extracted from red blood cells; however, other enzymes are less readily collected or tested. Basically, an enzyme test involves adding a measure of the enzyme to a test tube containing the precursor it is supposed to modify. Both the production of modified precursor and the rate at which it appears can be measured using laboratory equipment. If a modified precursor is produced, the test indicates that the enzyme is doing its job. The rate at which the modified precursor is produced can be compared to a standard to measure the enzyme's efficiency. |
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