Synthetic substrates are very sensitive; they can detect very low enzyme activities and are often more sensitive than a corresponding natural substrate. On the other hand, they can be less selective, or, have less discrimination in their reactivities toward related enzymes compared to the natural substrate. There are steps a scientist can take to maximize sensitivity and specificity. If the specificity of the enzymatic activity to be measured is known then a substrate selectivity table which shows the cross-reactivity of the substrates with different enzymes, and the kinetic data, such as that provided in the Chromogenix catalog, can be helpful. If the specificity of the enzyme is unknown, a screening procedure can be applied. This involves comparing the rate of hydrolysis obtained with different substrates. The presence of contaminating enzymes must also be taken into account. To eliminate interference, an inhibitor can be introduced, the sample can be further diluted, or conditions can be found where the relative activities are optimized. For instance, S-2222 is selective for FXa, but also for trypsin. If a researcher wants to measure FXa, s/he can add an inhibitor to trypsin, such as soybean trypsin inhibitor. Temperature, pH, buffers, and ionic strength can all affect the rate of hydrolysis and must be considered. Substrate concentration is also important, and a concentration of 2 x Km is usually appropriate. A good substrate has a low Km, meaning maximum reaction velocity is achieved at a low substrate concentration. In other words, the enzyme has a high affinity for the substrate. A high Kcat is also desired, which means the enzyme has a high turnover rate with the substrate (fast reaction).