Comparison of the measurement of serum cotinine levels by gas chromatography and radioimmunoassay

Standard cotinine solutions, controls and human serum samples containing cotinine have been measured by both radioimmunoassay (RIA) and gas chromatographic (GC) techniques. Cross-checks on standards and controls showed good agreement. However, for samples containing greater than 50 ng ml-1 of cotinine, RIA gave results on average 60% higher than GC. Determinations by using RIA and GC on samples containing less than 7 ng ml-1 of cotinine gave no significant correlation. The importance of the age of the serum sample has been investigated, and it is suggested that the age may affect the determination when dealing with low levels of cotinine.