What Is A Phantom hCG And What Causes the False Positive Pregnancy Test

    A "Phantom hCG" is a positive blood pregnancy test without being pregnant. Patients with a phantom hCG will have a persistently and not significantly rising positive blood pregnancy test but the urine test is negative.

    Phantom hCG and phantom choriocarcinoma syndrome (pseudohypergonadotropinemia) refer to persistent mild elevations of hCG, leading physicians to treat patients with cytotoxic chemotherapy for choriocarcinoma when in reality no true hCG or trophoblast disease is present. 

    The hCG test, or pregnancy test, is one of the most common tests done in  laboratories. It is assumed that an hCG assay detects only hCG or hCGß. This is not always the case. hCG kits can detect a wide and varying range of different hCG-related molecules in serum or urine samples.

    These include:

    • hCG dissociation and degradation products, like the free ß-subunit, nicked hCG and
    • ß-subunit core fragment
    • hyperglycosylated hCG a variant of hCG with separate biological functions and
    • a pituitary gland-produced variant of hCG

    The reason for a phantom hCG is that some individuals react falsely positive to some substrate in the blood hCG test and thus they will display a consistently low positive blood pregnancy test even though they are not pregnant, but their urine test will be negative. This phantom hCG may lead to serious misdiagnosis and intervention, but can be detected with serial dilutions. Patients with phantom hCG have a positive blood hCG but a negative urine hCG test. Therefore, if there is a question, a urine test should be done together with the blood hCG test.