PIIINP is the amino terminal peptide of type III procollagen, released from the precursor peptide during the synthesis and deposition of type III collagen. PIIINP in the serum can be derived from the synthesis of new type III collagen or from the degradation of existing type III collagen fibrils. There is evidence that serum PIIINP measurement is an effective non-invasive test for the detection and monitoring of Methotrexate-induced liver fibrosis and cirrhosis, and serial measurements may reduce the need for liver biopsy. Dermatology patients with repeated normal levels of PIIINP are very unlikely to have significant liver damage from fibrosis or cirrhosis. Conversely a high serum PIIINP or series of high values may indicate that a liver biopsy is required and in some cases, that Methotresate should be withdrawn.