PINP is a trimeric peptide consisting of two type 1 procollagen-α1 chains and a procollagen-α2 chain which are bonded non-covalently. 

PINP has very low circadian and biological variation, is not affected by food intake, and is very stable in serum after venepuncture. 

CTX is a degradation product of the type I collagen. The latter is a triple helix made up of two α1-chains and one α2-chain. The chains have a helical section thanks to the repetition, in their primary structure, of three amino acids: Gly-X-Y in which Y is often a proline or a hydroxyproline increasing the molecule stability while X can be either a proline or lysine. Triple helix structures are stabilized by covalent links. These covalent links are called cross-links. CTX is thus a degradatoion product of type 1 collagen, containing a cross-lap.

Interleukin 6 (IL-6), promptly and transiently produced in response to infections and tissue injuries, contributes to host defense through the stimulation of acute phase responses, hematopoiesis, and immune reactions. Although its expression is strictly controlled by transcriptional and posttranscriptional mechanisms, dysregulated continual synthesis of IL-6 plays a pathological effect on chronic inflammation and autoimmunity.