Differential Diagnosis

The patient's presentation of vaginal spotting, left-sided abdominal pain, and left adnexal tenderness, combined with a positive urine pregnancy test and a serum beta human chorionic gonadotropin (β-hCG) level of 1,800 mIU/mL, suggests several possible diagnoses. These can be categorized as follows:

• Single Most Likely Diagnosis

  • Ectopic Pregnancy: This is the most likely diagnosis given the combination of a positive pregnancy test, abdominal pain, adnexal tenderness, and a β-hCG level that is lower than expected for a normal intrauterine pregnancy at this gestational age. The presence of vaginal spotting further supports this diagnosis, as it can be a symptom of ectopic pregnancy due to implantation of the embryo outside the uterus, often leading to bleeding.

• Other Likely Diagnoses

  • Miscarriage: Although the β-hCG level is provided, a miscarriage (spontaneous abortion) could still be considered, especially if the pregnancy is not viable. However, the presence of adnexal tenderness and the specific β-hCG level might lean more towards an ectopic pregnancy.
  • Ovarian Cyst: An ovarian cyst could cause abdominal pain and adnexal tenderness. However, the positive pregnancy test and the specific symptoms suggest that the primary issue is related to the pregnancy.

• Do Not Miss Diagnoses

  • Ruptured Ectopic Pregnancy: This is a medical emergency. Even though ectopic pregnancy is already considered the most likely diagnosis, it's crucial to identify if there's a rupture, as it requires immediate surgical intervention.
  • Appendicitis: Although less likely given the context of a positive pregnancy test, appendicitis can present with abdominal pain and could be confused with adnexal tenderness. It's a diagnosis that cannot be missed due to its potential for serious complications if not treated promptly.
  • Ovarian Torsion: This is another emergency that could present with sudden onset of severe pelvic pain and could be associated with adnexal tenderness. It's less likely given the pregnancy context but is critical not to miss.

• Rare Diagnoses

  • Heterotopic Pregnancy: This is a rare condition where there is both an intrauterine and an ectopic pregnancy. It's more common in women undergoing fertility treatments but is still a rare occurrence. Given the symptoms and β-hCG level, this would be less likely but should be considered, especially if the patient has undergone fertility treatments.
  • Corpus Luteum Cyst: These cysts can form in response to hormonal changes during early pregnancy and might cause adnexal tenderness and pain. However, they are typically not as symptomatic as described and would not fully explain the combination of symptoms and laboratory findings presented.