catflap.org Online Dictionary Query

  Screening \Screen"ing\ (skr[=e]n"[i^]ng), n.
     the process of examining or testing objects methodically to
     find those having desirable properties. See screen[3].
  
     Note: In the pharmaceutical industry, pharmaceutical
           screening involves testing a large number of samples of
           substances to find those having desirable
           pharmacological activity; those samples which have the
           property sought are called active or positive in the
           screen. The substances tested may be pure compounds
           with known structure, mixtures of pure compounds, or
           complex mixtures obtained by extraction from living
           organisms. There are often additional sets of test
           performed on active samples, called
  
     counterscreening to eliminate those samples that may also
        possess undesirable properties. In the case of screening
        of mixtures from living organisms, a type of
        counterscreening called dereplication is usually
        performed, to determine if the active sample contains a
        known compound which has previously been studied.
        [PJC]

  Dereplication \De`rep*li*ca"tion\, n. (Biochem.)
     the process of testing samples of mixtures which are active
     in a screening process, so as to recognize and eliminate from
     consideration those active substances already studied; -- a
     stage subsequent to the preliminary screening in the process
     of discovery of new pharmacologically active substances in
     mixtures of natural products; -- also called
     counterscreening. See screening.
  
     Note: In the process of pharmaceutical screening (testing a
           large number of substances to find those having
           desirable pharmacological activity), the testing of
           samples of substances extracted from living organisms
           (plants, microorganisms, etc.) often detects substances
           already detected in prior screening. Such ``known'' or
           ``replicate'' activities must be recognized at an early
           stage to avoid duplicating previous efforts at
           purification and structural identification. The process
           of testing an sample which is active in a primary
           screen, to determine if the activity is due to a
           previously known substance, is called dereplication or
           counterscreening.
           [PJC]