The concept of a ‘discriminatory β-hCG level’ was introduced in 1985 to highlight the serum concentration of β-hCG when a pregnancy should be visible on an ultrasound scan. Using transabdominal ultrasound examination, it was reported then that the absence of an intrauterine gestational sac at a β-hCG concentration over 6500 IU/l had a sensitivity of 100%, specificity of 96%, positive predictive value of 87% and negative predictive value of 100% for the prediction of ectopic pregnancy.