Introduction
Endometriosis is defined as the presence of endometrial glands and stroma outside the uterus, and is characterized by pain and subfertility¹. At present, laparoscopy is considered the gold standard diagnostic test, preferably with histological confirmation². Symptoms and fertility status should guide patient management rather than biomarker testing. However, serum CA-125 has been used as a biomarker for the disease and serial levels can reflect disease progression or response to treatment³. CA-125 is a well-established glycoprotein biomarker for epithelial cell ovarian cancer and is derived from coelomic epithelia which includes the endometrium, fallopian tube, ovary and peritoneum⁴. Increased serum CA-125 levels are found with endometriosis³, particularly deep infiltrating endometriosis⁵ and endometriomata⁶. A recent systematic review of the diagnostic accuracy of CA-125 in endometriosis found a pooled specificity of 93% and sensitivity of 52%¹. Cyclical variation in serum CA-125 levels has been reported, with highest levels detected during menses in women with and without endometriosis⁷. Even after laparoscopic diagnosis and treatment, levels continue to be elevated during menses⁸. We report two cases of women with endometriosis, who underwent serial serum CA-125 measurements within the same menstrual cycle: one test was performed during menses on day three and a second was performed mid-cycle.

Case reports
Case 1
A 40-year-old woman with primary subfertility was attending for regular surveillance because of endosalpingiosis diagnosed at laparoscopy two years earlier. She also had endometriosis. As endosalpingiosis has been associated with ovarian cancer⁹, she was being monitored with three to six monthly CA-125 testing. Over the two years, levels varied between 58 and 67 units/ml. The index test showed a level of 163 units/ml which was of concern due to the elevation. It was noticed, however, that this had been taken during menstruation. A repeat test on day 17 of her cycle gave a result of 69 units/ml.

Case 2
A 36-year-old was referred from her GP with secondary subfertility, severe dysmenorrhoea, and a raised CA-125 level of 554 units/ml. The next available date for surgery was three months hence and there was concern that emergency surgery was indicated because of the CA-125 level. The initial test had been taken during menses. When repeated on day 12 of her menstrual cycle, her CA-125 level was 262 units/ml. She underwent laparoscopy six weeks after her second test, and was diagnosed with stage four endometriosis with an endometrioma.

Discussion
Our findings demonstrate the significant variation in serum CA-125 levels taken during menstruation and at other times during a woman’s menstrual cycle. These findings have implications for serum CA-125 testing during menses in women with suspected endometriosis, as testing during this time may result in falsely elevated results. Pittaway et al. reported that CA-125 antigen levels increased in women with and without endometriosis during menses⁷. A case report of presumed endometriosis described a ten-fold increase in CA-125 levels during menses compared to the rest of the cycle (1113+-185.8 units/ml vs. 108+-60.0 units/ml; p<0.001)¹⁰.

Kafali et al. compared serum CA-125 levels three months after laparoscopy in 16 women diagnosed with endometriosis and 12 women without. In women with endometriosis, there was a 198.3% difference in CA-125 levels between menses and mid-cycle days 10-15, compared with a 22% difference in women with no endometriosis (p<0.001)⁸. These authors suggested that this difference could be used as a diagnostic tool - a sensitivity of 93% and specificity of 92% for the diagnosis of endometriosis was achieved, using a threshold of an 83% increase in CA-125 during menses.

Our cases are an important addition to the literature regarding raised serum CA-125 levels in women with endometriosis. The high levels found during menstruation can cause unnecessary concern for patients, and their doctors. We recommend that CA-125 testing in women with suspected endometriosis be performed outside the menstrual phase of the cycle.

Conflicts of Interest
The authors have no conflicts of interest to declare.

Correspondence:
Dr. David Crosby MB BCh MAO, MRCPI, MRCOG, Merrion Fertility Clinic, 60 Mount Street, Dublin 2
Tel: 353 1 663500
E mail: [email protected]

References
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8) Kafali H, Artuc H, Demir N. Use of CA125 fluctuation during the menstrual cycle as a tool in the clinical diagnosis of endometriosis; a preliminary report. Eur J Obstet Gynecol Reprod Biol. 2004 Sep 10;116(1):85-8.
9) Esselen KM, Terry KL, Samuel A, Elias KM, Davis M, Welch WR, Muto MG, Ng SW, Berkowitz RS. Endosalpingiosis: More than just an incidental finding at the time of gynecologic surgery? Gynecol Oncol. 2016 Aug;142(2):255-60
10) Imai A, Horibe S, Takagi A, Takagi H, Tamaya T. Drastic elevation of serum CA125, CA72-4 and CA19-9 levels during menses in a patient with probable endometriosis. Eur J Obstet Gynecol Reprod Biol. 1998 May;78(1):79-81.

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