The pressure of an antibiotic in a fluid is determined by contacting a sample of the fluid suspected of containing an antibiotic with a test medium containing a microorganism, a substance that provides a solid state and an indicator, such as Bromothymol Blue, incubating to grow the microorganism if present in the sample and detecting growth of the microorganism indicating absence of an antibiotic or inhibition of the growth of microorganism indicating the presence of an antibiotic.