Endometriosis Blood Test: What Blood Tests Can (and Can't) Tell You

Endometriosis affects an estimated 1.5 million women and people assigned female at birth in the United Kingdom. It is a chronic condition in which tissue similar to the lining of the uterus (the endometrium) grows outside the uterus — most commonly on the ovaries, fallopian tubes, pelvic peritoneum, and bowel. This ectopic tissue responds to the menstrual cycle hormones, thickening, breaking down, and bleeding each month, but unlike the uterine lining, it has no way to exit the body. The result is chronic inflammation, adhesion formation, and often severe pain.

The average diagnostic delay for endometriosis in the UK is 8 years. Eight years of symptoms being dismissed, misdiagnosed as irritable bowel syndrome, or attributed to "normal period pain." This delay is one of the most significant failures in modern gynaecological care, and it persists in part because there is no simple, non-invasive diagnostic test.

Can a Blood Test Diagnose Endometriosis?

No. This needs to be stated clearly and honestly. No currently available blood test can definitively diagnose endometriosis. The gold standard for diagnosis remains laparoscopy — a surgical procedure in which a camera is inserted through a small incision in the abdomen to directly visualise and biopsy endometriotic lesions. More recently, MRI and transvaginal ultrasound performed by experienced specialists have improved non-invasive detection, particularly for deep infiltrating endometriosis and ovarian endometriomas (chocolate cysts).

However, blood tests do have a role. They can support a clinical suspicion of endometriosis when interpreted alongside symptoms and imaging. They can identify the metabolic consequences of the condition — iron deficiency from heavy menstrual bleeding, chronic inflammation, hormonal imbalances. And they can help rule out other conditions that mimic endometriosis symptoms. Understanding what blood tests can and cannot tell you about endometriosis allows you to use them appropriately, without false expectations.

CA-125: The Most Studied Blood Marker

CA-125 (cancer antigen 125) is a glycoprotein found on the surface of cells derived from the coelomic epithelium — which includes the peritoneum, pleura, pericardium, and the endometrium. It is best known as a tumour marker for ovarian cancer, but it is elevated in a range of benign conditions, including endometriosis.

The evidence on CA-125 in endometriosis:

• Sensitivity: Elevated (above 35 U/mL) in 50-80% of women with moderate to severe endometriosis (stages III-IV), but only 25-50% of those with minimal to mild disease (stages I-II)
• Specificity: Moderate at best. CA-125 is also raised in ovarian cancer, pelvic inflammatory disease, adenomyosis, fibroids, menstruation, pregnancy, and liver disease
• Diagnostic utility: A Cochrane review concluded that CA-125 does not have sufficient sensitivity or specificity to be used as a standalone diagnostic test for endometriosis

So when is CA-125 useful? In practice, a significantly elevated CA-125 (above 50-100 U/mL) in a woman with classic endometriosis symptoms (cyclical pelvic pain, dysmenorrhoea, dyspareunia, heavy periods, subfertility) adds weight to the clinical picture and may strengthen the case for specialist referral or diagnostic laparoscopy. It is most useful as a supporting marker, not a screening or diagnostic test. Serial measurements can also help monitor treatment response — falling CA-125 levels after hormonal treatment or surgery suggest disease regression.

Inflammatory Markers: CRP and ESR

Endometriosis is fundamentally an inflammatory condition. The ectopic endometrial tissue provokes a chronic inflammatory response in the pelvis, with elevated levels of pro-inflammatory cytokines (IL-1, IL-6, TNF-alpha) detectable in peritoneal fluid. Whilst CRP and ESR are non-specific and often remain within the normal range in mild endometriosis, moderately elevated CRP (above 3-5 mg/L) in a woman with pelvic pain symptoms can support the suspicion of an inflammatory process.

More importantly, CRP and ESR help differentiate endometriosis from pelvic infection. Acute pelvic inflammatory disease (PID) typically produces markedly elevated CRP (often above 50 mg/L) and ESR, whereas endometriosis tends to produce either normal or mildly elevated inflammatory markers. This distinction is clinically relevant because the management of PID (antibiotics) is entirely different from that of endometriosis.

Iron and Ferritin: The Consequence of Heavy Bleeding

Heavy menstrual bleeding (menorrhagia) is one of the hallmark symptoms of endometriosis, particularly when adenomyosis (endometrial tissue growing into the muscular wall of the uterus) coexists. Chronic heavy periods deplete iron stores progressively, often producing iron deficiency months or years before haemoglobin drops below the anaemia threshold.

The pattern to look for:

• Low ferritin (below 30 µg/L): Iron depletion — common in women with heavy periods
• Low ferritin with normal haemoglobin: Iron depletion without anaemia — causes fatigue, poor concentration, and hair thinning despite "normal" blood counts
• Low ferritin with low haemoglobin: Iron deficiency anaemia — the most advanced stage

Ferritin should be tested in any woman with suspected endometriosis. Treating iron deficiency can significantly improve quality of life even before the underlying endometriosis is definitively managed. Oral iron supplementation is the first-line treatment, though some women with malabsorption or severe depletion may require intravenous iron.

Hormone Panel: Oestradiol and Progesterone

Endometriosis is an oestrogen-dependent condition. The ectopic tissue contains oestrogen receptors and is stimulated by circulating oestradiol. In fact, endometriotic implants can produce their own oestrogen through local aromatase activity, creating a self-sustaining inflammatory and proliferative cycle.

Measuring oestradiol and progesterone can be informative in several ways:

• Oestradiol levels: Whilst they do not diagnose endometriosis, they provide context for symptom severity. Women with higher circulating oestradiol may experience more aggressive endometriosis symptoms.
• Progesterone: Measured in the luteal phase (typically day 21 of a 28-day cycle), progesterone confirms ovulation. Progesterone also has anti-inflammatory and anti-proliferative effects on endometrial tissue. Relative progesterone deficiency (or progesterone resistance, which is documented in endometriosis) may contribute to disease progression.
• Treatment monitoring: Hormonal treatments for endometriosis — combined oral contraceptives, progestogens, GnRH agonists — work by suppressing oestradiol levels or opposing its effects. Hormone measurements can confirm that treatment is achieving the desired hormonal suppression.

Thyroid Function

There is growing evidence of an association between endometriosis and autoimmune thyroid disease. A 2019 meta-analysis published in the European Journal of Obstetrics, Gynecology, and Reproductive Biology found that women with endometriosis had a significantly higher prevalence of autoimmune thyroiditis (Hashimoto's disease) compared with controls. The proposed mechanism involves shared immune dysregulation — the same aberrant immune processes that permit ectopic endometrial tissue to survive and proliferate may also contribute to autoimmune thyroid destruction.

Given this association, thyroid function (TSH, free T4) and anti-TPO antibodies should be considered in women with endometriosis who report fatigue, weight changes, or other thyroid-related symptoms. Hypothyroidism can compound the fatigue already caused by chronic pain and iron deficiency, creating a triple burden that is treatable once identified.

Full Blood Count (FBC)

A full blood count provides essential baseline data:

• Haemoglobin: Detects frank anaemia from chronic heavy periods
• MCV (mean corpuscular volume): Low MCV with low haemoglobin confirms microcytic anaemia, most likely from iron deficiency
• White blood cell count: Helps differentiate inflammatory from infectious causes of pelvic pain
• Platelets: Reactive thrombocytosis (elevated platelets) can accompany chronic iron deficiency and inflammation

What These Tests Cannot Do

Honesty about limitations is essential. Blood tests cannot:

• Confirm or rule out endometriosis with certainty
• Determine the stage or extent of endometriotic lesions
• Replace imaging (ultrasound, MRI) or laparoscopy for definitive diagnosis
• Detect superficial peritoneal endometriosis, which is the most common form and often invisible on imaging

What blood tests can do is build a supporting picture. A woman presenting with cyclical pelvic pain, dysmenorrhoea, and dyspareunia, who also has elevated CA-125 (60 U/mL), low ferritin (12 µg/L), mildly elevated CRP, and positive anti-TPO antibodies, has a clinical profile that strongly warrants specialist gynaecological referral. The combination of symptoms and biomarkers is far more powerful than any single test in isolation.

Testing With Lola Health

Our Core Health 45 blood test (£120) covers a full blood count, iron studies, ferritin, CRP, thyroid function, and a comprehensive metabolic panel — providing the foundational blood work for investigating endometriosis-related symptoms. CA-125 is available as an add-on biomarker, which we recommend for women with suspected endometriosis who want to include this marker in their panel.

All results are reviewed by qualified clinicians before release. If your results reveal iron deficiency, thyroid dysfunction, or elevated inflammatory markers, our medical team will provide detailed commentary and recommendations — whether that means starting supplementation, requesting a GP referral for gynaecological assessment, or scheduling follow-up testing.

Advocating for Yourself

The 8-year diagnostic delay for endometriosis is not inevitable. Arriving at your GP appointment with a comprehensive blood panel already completed — showing your iron status, inflammatory markers, hormone levels, and thyroid function — demonstrates that you have done your groundwork and provides objective data to support your case for specialist referral. Many women report that having concrete test results in hand makes the conversation with their doctor more productive and harder to dismiss.

Endometriosis cannot yet be diagnosed with a blood test. But blood tests can identify the metabolic consequences of the condition, support the clinical suspicion, rule out alternative diagnoses, and — perhaps most importantly — give you the evidence you need to push for the specialist care you deserve.

Emerging Research

The search for a reliable blood-based diagnostic test for endometriosis is one of the most active areas of gynaecological research. MicroRNA panels, cytokine profiles, and novel biomarker combinations are being investigated in clinical trials. A saliva-based microRNA test has shown promising results in early studies, with reported sensitivity exceeding 95%. If validated in larger trials, such tests could eventually replace laparoscopy for diagnosis in many cases. For now, the markers described above represent the best available blood-based approach, used alongside clinical assessment and imaging.