What to do when an at-home Human Chorionic Gonadotropin (HCG) test is positive but an in-office test is negative, should I retest?

Discrepant hCG Test Results: Management Approach

When an at-home pregnancy test is positive but an in-office test is negative, a repeat test should be performed using a different hCG assay, as different assays have varying sensitivities and may detect different forms of hCG. 1

Understanding the Discrepancy

• Different hCG assays may have varying sensitivities and specificities; using the same laboratory for serial measurements is recommended 2
• Home pregnancy tests vary widely in detection limits for regular hCG (6.3-50 IU/L), and 60% of devices have poorer detection limits for hyperglycosylated hCG (H-hCG), which is the principal hCG form in early pregnancy 3
• Several commercial hCG assays have problems with false-positive or false-negative results due to their ability to detect different hCG isoforms/fragments 1

Recommended Approach

1. Repeat testing with a different assay:

   • When hCG results do not fit the clinical picture, measure the hCG on a different assay 1
   • Use a laboratory test specific for intact hCG which can reduce false-positive results by up to 92% 4

2. Consider urine testing:

   • When a false positive is suspected in serum, assessment of urine hCG can be helpful 1
   • Cross-reactive molecules in blood that cause false positives rarely get into urine, so a positive urine hCG excludes a false-positive serum result 1
   • Quantitative urine hCG testing can be critical in resolving discrepant results 5

3. Timing considerations:

   • Most qualitative pregnancy tests require an additional 11 days past the expected menses to detect 100% of pregnancies 2
   • Early pregnancy may have hCG levels below the detection threshold of some office-based tests 1

Special Considerations

• Potential causes of false-positive home tests:

  • Assay-interfering molecules such as human antimouse antibodies 6
  • Recent pregnancy termination (spontaneous or induced) as hCG can remain detectable for several weeks 2
  • Certain medications containing hCG 7

• Potential causes of false-negative office tests:

  • Very early pregnancy with low hCG levels 1
  • Use of an assay that doesn't detect the specific forms of hCG present 1
  • Technical issues with the test procedure 2

• Non-pregnancy causes of elevated hCG:

  • Gestational trophoblastic disease (GTD) including hydatidiform mole 8, 6
  • Certain malignancies that produce hCG 6
  • Pituitary hCG production in perimenopausal or menopausal women 9

Follow-up Recommendations

• If repeat testing remains discrepant, consider:

  • Transvaginal ultrasound to evaluate for intrauterine pregnancy, especially if hCG levels are above 1,000 mIU/mL 1
  • Serial hCG measurements 48 hours apart to assess for appropriate rise (typically doubles every 48-72 hours in viable intrauterine pregnancy) 1
  • Evaluation for potential gestational trophoblastic disease if hCG patterns are abnormal 8, 6

• For perimenopausal women (41-55 years) with low positive hCG (5.0-14.0 IU/L), check FSH levels; pregnancy is unlikely if FSH >20.0 IU/L 9