Currently, diagnosis in unruptured ectopic pregnancy is achieved using a combination of transvaginal ultrasonography and measurement of serum β-hCG concentrations. One of the key elements in the diagnosis is the exclusion of a viable or non-viable IUP. Diagnosis can be straightforward when a transvaginal ultrasound scan (TVS) positively identifies an IUP or ectopic pregnancy (Figure 1). However, TVS fails to identify the location of a pregnancy in a significant number of women and such women are currently diagnosed as having a ‘pregnancy of unknown location’ (PUL).
The 2006–2008 CMACE report drew attention to a maternal death secondary to ruptured ectopic pregnancy where a diagnosis of PUL had been made. Although most patients with a PUL will subsequently be diagnosed with either a failed IUP (a spontaneous abortion) or viable IUP, the report highlights that 7-20% will be diagnosed with an ectopic pregnancy. It is therefore very important that a diagnosis of PUL should trigger further diagnostic pathways and follow-up until the final outcome of the pregnancy is known.
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. In the context of a 19.4% prevalence of ectopic pregnancies in the study group, this diagnostic paradigm was 98% efficient. With the introduction of high-resolution TVS, the discriminatory β-hCG level of 6500 IU/l is now less helpful. An ectopic pregnancy can be detected at β-hCG concentrations well below this level and an ultrasound scan should not be delayed because of low β-hCG concentrations.