A positive result of Hoesch Test is shown in the image. This test allows to confirm or rule out an acute attack of Porphyria. It is a simple, fast and cheap method that produces the immediate color of the urine in contact with Ehrlich reactive. This reactive can be easily prepared in the laboratory adding 42 ml of acetic acid (100%) and 8 ml of percloric acid (60%) to 1 grame of 4-dimetilamine-benzaldehyde. Hoesch test is done by addition of 2-3 drops of the urine to 1 ml of reactive in suspected cases of acute Porphyria.

Figure 1. Lithiasis on suture thread in a 79-year-old man. A) and B) Lithiasis on suture thread. C) Fragment: 55% whewellite, 40% weddellite and 5% hydroxyapatite. D) Infrared spectrum (IRS) of organic matter. E) IRS corresponding to whewellite, weddellite, and hydroxyapatite.

The main cause of lithogenesis is supersaturation of some constituents in urine. Other factors as extreme pH, anatomical malformations or presence of promoters can facilitate this process too.