The use of serum biochemistry

Serum biochemistry is generally used most for the management of pregnancy of unknown location or PUL. This is discussed in more detail in an accompanying subsection of this site. However it is important to understand that biochemical tests cannot be used to locate a pregnancy, and the diagnosis of an ectopic pregnancy should never be made based on a rising hCG without positive identification of the site of the pregnancy either by ultrasound or laparoscopy. In general serum hCG or progesterone can be used to determine the viability of a pregnancy, but can only give an indication regarding the location. There is no hCG threshold above which one should expect to see an ectopic pregnancy or a failing intrauterine pregnancy on scan. Perhaps most important there is no lower limit of hCG below which an ectopic pregnancy is unlikely. So patients should not be prevented from having a scan because the serum hCG is below a certain level such as 1000 iu/l or 500 iu/l. This can be seen in the table below taken from:

Condous G, Kirk E, Lu C, Van Huffel S, Gevaert O, De Moor B, De Smet F, Timmerman D, Bourne T. Diagnostic accuracy of varying discriminatory zones for the prediction of ectopic pregnancy in women with a pregnancy of unknown location. Ultrasound Obstet Gynecol. 2005 Dec;26(7):770-5