Table 2.

Calculation of likelihood ratios

Test score	Positive fragile X PCR test, no. of patients	Negative fragile X PCR test, no. of patients	Likelihood ratios
5	12	129	for positive test: (12/12)/(129/323) = 2.5
<5	0	194	For negative test: (0/12)/(194/323) = 0
Total	12	323
PCR = polymerase chain reaction