Porphyrins are tetrapyrroles (composed of four pyrroles). Porphyrins in their oxidized forms are reddish in color and are also fluorescent. Fluorescent substances, when exposed to light at certain wavelengths, emit light with a different wavelength. Porphyrins appear intensely red when exposed to long-wave ultraviolet light (UV-A). This makes them visible with a Wood's lamp, and enables them to be measured accurately with a spectrofluorometer. Within cells, all porphyrins that are intermediates in the heme biosynthetic pathway, with one exception, are in the reduced form, and are colorless and nonfluorescent. The last intermediate, protoporphyrin, is an oxidized porphyrin. Porphyrins that leave the cells and appear in blood, urine and feces are mostly oxidized and appear reddish to the naked eye and are fluorescent.

The total amount of porphyrins in a urine sample is easily measured. This is a useful test for screening especially when combined with ALA and PBG. But an increase in urine porphyrins is nonspecific, and may not be an indication of an acute Porphyria if ALA and PBG are normal.

A variety of porphyrins are present in urine. When there is an increase, particularly a large increase, in total urine porphyrins, it is often useful to determine the individual porphyrins found in urine. It is seldom important to do this if the total in normal.

The most common method for separating the individual porphyrins is "high performance liquid chromatography" (HPLC). This method will measure amounts of porphyrins with 4 or more carboxyl groups found in urine (see Table 3). In interpreting HPLC results, it is most useful to see which porphyrins predominate rather than focus on the amounts of each porphyrin.

Urinary porphyrins are oxidized intermediate metabolites of heme biosynthesis and are readily excreted in excess when porphyrinogens accumulate as a result of inhibition of specific enzymes in the heme biosynthetic pathway. Heme is required for oxygen binding, transport and utilization, cytochromes, and electron transport in mitrochondira. The high rate of production of heme facilitates the use of urinary porphyrins as early and sensitive biomarkers of disorders in heme production, which has long been associated with genetic disorders, metabolic disturbances and diseases, nutritional status, oxidative stress and high-level exposure to toxic chemicals or metals.