We generally require one double-dose cell for rule-outs (from a homozygous donor--sorry, I know John Judd still lurks). For K, we accept 2 single-dose cells. For C and E in the presence of anti-D, we accept one single dose cell for rule out. The 3 pos and 3 neg rule applies to the statistical likelihood that a particular set of reactions could occur by chance rather than due to the presence of a specific antibody. By requiring 3 pos and 3 neg (or 2 pos and 5 neg) TO IDENTIFY AN ANTIBODY (not to rule one out) you are statistically pretty likely that the ID is accurate and not due to chance. These are the basic rules taught to our bench techs. More complicated cases, like multiples and antibodies to low-incidence antigens are handled by experienced staff on a case-by-case basis and occasionally some of the rules are not applied to the same degree. Of course, Lewis can't actually have double dose cells, but they all look like they are double-dose on the panel, so it makes the rules appear consistent to the average generalist.