Introduction

Measurement of human chorionic gonadotrophin (hCG) is the key test in the differential diagnosis of abdominal pain in women of reproductive age. Advances in assay methodology have resulted in the marketing of a wide variety of pregnancy testing kits for home or bedside use. These are summarised in Box 3.1. Measurement of hCG is of particular value in women at high risk for ectopic pregnancy (Group 2) and those with a subacute presentation (Group 3). The current approach to evaluation of the clinically stable patient with a possible ectopic pregnancy also includes an ultrasound examination and occasionally laparoscopy to locate the pregnancy.

The false negative rate of hCG estimation depends on the cut-off level or detection limit of the assay. The lower the detection limit, the fewer the ectopic pregnancies that are missed. State-of-the-art urine hCG assays typically have a detection limit of 25-50IU/L with false negative rates of less than 2%. Earlier assays with detection limits of 100-500 IU/L should not be used for this indication (Grudzinskas and Stabile, 1993). Whatever the detection limit of the assay, the clinician should maintain a high index of suspicion for ectopic pregnancy even if hCG is undetectable.

Important questions about the use of biochemical tests in ectopic pregnancy remain unresolved. What is the cut-off level most useful in the diagnosis of ectopic pregnancy? Are serial quantitative estimations of greater value than single measurements? Is there is any advantage in multiple biochemical tests? Should they be combined with ultrasound?