In the standardized method outlined above, the inoculum is prepared from colonies on a primary culture plate or from a pure culture. This is called an “indirect sensitivity test”. In certain cases, where a rapid answer is important, the standardized inoculum may be replaced by the pathological specimen itself, e.g., urine, a positive blood culture, or a swab of pus. For specimens of urine, a microscopical examination of the sediment should first be made in order to see if there is evidence of infection, i.e., the presence of pus cells and/or organisms. The urine may then be used as the inoculum in the standard test. Likewise, susceptibility tests may be performed on incubated blood cultures showing evidence of bacterial growth, or a swab of pus may be used as a direct inoculum, when a Gram-stained smear shows the presence of large numbers of a single type of organism. This is called a “direct susceptibility test”; its advantage over the indirect test is that a result is obtained 24 hours earlier. The main disadvantage is that the inoculum cannot be properly controlled. When the susceptibility plate shows too light or too heavy growth, or when the culture is a mixture, the results should be interpreted with caution, and the test repeated on pure cultures.