You can
1) find the mean across several variables using mean(t{:, ["Var1", "Var2", "Var3"]}, 2)
2) find rows that meet a certain condition (e.g. absent for both tests) by using logical indexing such as t{t.Condition1 & t.Condition2, "Mean"} = missing.
t = array2table(randi(10, 3, 4))
t.Mean = mean(t{:, ["Var1", "Var2", "Var3"]}, 2)
t{t.Var1 > 5 & t.Var2 > 5, "Mean"} = missing