<i>Background:</i> We have developed a rapid, sensitive and quantitative in vitro assay for leptin based upon its ability to bind to the soluble extracellular domain of the leptin receptor (sOB-R). Such an assay is theoretically capable of differentiating between physiologically active leptin molecules from those with modified, either enhanced or reduced, binding activity. <i>Methods:</i> A preparation of sOB-R was immobilized to capture leptin from serum samples or standards. Anti-leptin antibodies that had been raised in rabbits were added in a second incubation step to identify leptin molecules bound to sOB-R. Signal detection was performed in a third incubation step by anti-rabbit IgG labeled with peroxidase. The immunofunctional assay (IFA) was clinically validated by the comparison of leptin levels in adolescents (n = 41, age range 9–18 years, BMI range 13.4–33.8 kg/m<sup>2</sup>) and adults (n = 80, age range 18–77 years, BMI range 16.4–54.7 kg/m<sup>2</sup>) measured using the IFA with data of an in-house RIA performed with the same standards and leptin antibodies. <i>Results:</i> The functional sensitivity of the IFA was 0.4 ng/ml and comparable to the data of the RIA. Intra- and interassay coefficients of variation were below 12.5 % in both methods. Leptin levels correlated well with the BMI of the subjects studied (r = 0.70 for RIA, r = 0.72 for IFA; p < 0.0001) as well as between IFA (y) and RIA (x) (y = x –1.31 ng/ml; r = 0.97, p < 0.0001). The median of the quotient between IFA and RIA levels was 0.86 (quartile range 0.60–1.10) for all samples.<i></i><i>Conclusions:</i> So far, only at the most minor differences between leptin measurements using the newly developed IFA and those using a conventional RIA have been detected. Additional studies using the IFA method are required to investigate whether or not discrepant results with the IFA will be seen in various states of relative leptin resistance and whether or not such differences are of biological relevance.

Kratzsch, J., Berthold, A., Lammert, A., Reuter, W., Keller, E., & Kiess, W. (2002). A Rapid, Quantitative Immunofunctional Assay for Measuring Human Leptin. Hormone Research in Paediatrics, 57(3–4), 127–132. Portico.