Abstract: The present invention is directed to the measurement of soluble T cell growth factor receptors, soluble T cell differentiation antigens, or related soluble molecules or fragments thereof, and the use of such measurements in the diagnosis, staging, and therapy of diseases and disorders. Specific embodiments involve the diagnosis and monitoring of therapy using absolute values of such soluble molecules. Further embodiments involve detecting a change in the levels of such soluble molecules, in the diagnosis and therapy of diseases and disorders. In specific embodiments, measurements of interleukin-2 receptor levels can be made to detect lung cancer, or to stage squamous cell lung carcinoma. In other embodiments, detection of increases in both soluble IL2R and creatinine in the body fluid of a transplant patient can be used to differentially diagnose renal allograft rejection from infection.