In the bloodstream the majority of steroid hormones are bound either to non-specific proteins such as albumin or to specific proteins such as corticosteroid binding globulin (CBG) or sex hormone binding globulin (SHBG).  These protein molecules are generally too large to diffuse from the blood stream into the saliva glands.  Consequently, only the 1-15% of circulating hormone molecules that are found in an unbound state are free to diffuse from blood into saliva.  Steroid hormone concentrations in saliva are therefore much lower than those in blood. If the barrier between the bloodstream and the oral mucosa is compromised by inflammation or microinjury such that there is leakage of blood or plasma into saliva, then there is a possibility that the higher levels of hormones found in the blood stream will contaminate saliva samples, causing falsely high readings. (1) Visual inspection of saliva samples is not adequate to detect blood contamination, since microinjuries to blood vessel membranes may allow hormone molecules and proteins to pass through while still retaining the much larger red blood cells.  Dipstick tests for blood, which look for the presence of hemoglobin, are also not a reliable means of screening saliva samples, due to the presence of peroxidases in saliva, which can generate false positive results. (2,3) The Salimetrics Salivary Blood Contamination Enzyme Immunoassay quantitatively measures transferrin, a large protein (mol wt 76,000) present in abundance in blood that is normally present in only trace amounts in saliva.   Higher levels of transferrin measured in saliva by this assay indicate the presence of blood contamination and serve as a warning to investigators that samples should be excluded from subsequent quantitative assays for salivary analytes and statistical analyses.  (2,3,4)  Saliva samples collected from populations that have little or no dental care, or known oral health problems, are especially appropriate for screening with the Blood Contamination Assay Kit.