Here you go-
The half life of Lasix is around 2 hours so it's not around long.


Immunoassay detection of drugs in racing horses. VI. Detection of furosemide (Lasix) in equine blood by a one step ELISA and PCFIA.
Woods WE1, Wang CJ, Houtz PK, Tai HH, Wood T, Weckman TJ, Yang JM, Chang SL, Blake JW, Tobin T, et al.
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Abstract
A one step enzyme-linked immunosorbent assay (ELISA) and a particle concentration fluorescent immunoassay (PCFIA) test for furosemide were evaluated as part of a panel of pre- and post-race tests for illegal medication of racing horses. These tests are very sensitive to furosemide with an I-50 for furosemide of about 20 ng/ml. The test is also rapid; an average pre-race complement of 10 samples can be analyzed in 90 minutes or less. The ELISA test results can be read with an inexpensive spectrophotometer, or even by eye. Both the PCFIA test and the ELISA test readily detect the presence of furosemide in equine blood for up to five hours after administration of the recommended therapeutic dose of this agent. The principal utility of these tests lies in rapid screening of samples for compliance with regulations governing the use of furosemide. Thus these tests can be used pre-race to determine whether horsemen have treated their horses with furosemide, and post-race to perform an initial evaluation of whether certain blood concentrations of furosemide have been exceeded. Pilot trials with these systems in Kentucky and Illinois suggest that these tests are economical and effective, and can form part of an analytical approach to substitute for the detention barn system of monitoring furosemide administration.