The whole purpose of this biochemical pathway is to produce haem for the body's needs. The porphyrins are indicated in the centre in yellow. You can see how each intermediate compound is converted to the next under the influence of an enzyme , named on the left in blue. The porphyria  resulting from a defect in that enzyme is shown on the right in green. The step between uroporphyrinogen and coproporphyrinogen includes three intermediates: hepta-, hexa- and pentacarboxylic porphyrinogens (abbreviate here as C7-, C6- and C5-): accumulation of these intermediates is a feature of porphyria cutanea tarda. The laboratory analyses the pattern of porphyrins which accumulate in the urine, plasma and sometimes the stool or red blood cells of a patient to determine just what type of porphyria they have: the process is known as chromatography, and this is the basis of the biochemical tests employed in diagnosis.

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