An in vitro (laboratory) test of the interaction between sperm and cervical mucus, in which the mucus to be tested is drawn into two fine glass tubes (another two tubes are filled with mucus that is known to be good) to give a 4-way test between: husband's (or male partner's) sperm and wife's (or female partner's) mucus; husband's sperm and known good mucus; known good sperm with wife's mucus; known good sperm with known good mucus. Because of the laboriousness of the test it's usual to administer estrogen for at least a week in the follicular phase to ensure that the mucus is optimal.