Heme (component of hemoglobin in red blood cells that carries oxygen throughout the body) is broken down into bilirubin, which moves to the liver where it is processed and added to bile, a digestive fluid.
When examined under a microscope, the RBCs also appear pale or light colored from the absence of heme, the major component of hemoglobin, which is the iron-bearing protein and coloring pigment in RBCs.
Enzymes involved in heme biosynthesis display subtle, tissue-specific variations; therefore, heme biosynthesis may be impeded in the liver, but normal in the immature red blood cells, or vice versa.
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.
Other biochemical tests rely on the fact that heme precursors become less soluble in water (able to be dissolved in water) as they progress further through the heme biosynthesis pathway.