Diagnosis basics

TNBC isn’t diagnosed in a single step. It’s identified through a sequence: first finding a suspicious lump or lesion, then confirming it’s cancer, then testing the cancer cells for the three markers that define the subtype.

Step 1: Detection of a suspicious finding

The process usually starts in one of three ways:

• Screening mammography picks up an abnormality before symptoms appear. TNBC can be harder to catch on mammograms than other subtypes because the tumors are often round and well-circumscribed, which can mimic benign cysts.
• A palpable lump or other symptom — many TNBC cases are found this way, in part because TNBC disproportionately affects younger women who aren’t yet in routine screening age ranges, and because of the imaging features above. Symptoms can include a lump, skin changes, nipple changes, or breast pain.
• High-risk surveillance — women with BRCA1 mutations or strong family histories often have MRI-based screening that may catch TNBC earlier.

Step 2: Diagnostic imaging

Once something suspicious is identified, imaging characterizes it more completely:

• Diagnostic mammography — more detailed views than screening
• Breast ultrasound — distinguishes solid masses from fluid-filled cysts and guides biopsy
• Breast MRI — sometimes used to assess tumor extent, especially in dense breast tissue or to look for additional lesions

These images don’t diagnose cancer; they assess how suspicious a finding is and guide the next step.

Step 3: Biopsy

A definitive diagnosis requires tissue. The standard approach is a core needle biopsy, in which a hollow needle (guided by ultrasound or mammography) removes small cylinders of tissue from the suspicious area. Less commonly, a surgical biopsy is performed. Fine-needle aspiration (a thinner needle that withdraws cells only) is generally not sufficient for full receptor testing.

Step 4: Pathology — confirming cancer and identifying the subtype

A pathologist examines the biopsy tissue under a microscope and performs molecular testing on the cancer cells. This is where TNBC is actually diagnosed. Two main techniques are used:

• Immunohistochemistry (IHC) — stains are applied that bind to specific proteins in the cancer cells. The pathologist looks for staining indicating estrogen receptor (ER), progesterone receptor (PR), and HER2 protein.
• In situ hybridization (ISH, usually FISH) — used as a follow-up when HER2 IHC results are ambiguous (a score of 2+). This test counts copies of the HER2 gene in the cancer cells.

A cancer is classified as triple-negative when:

• ER: less than 1% of cancer cell nuclei stain positive
• PR: less than 1% of cancer cell nuclei stain positive
• HER2: IHC score of 0 or 1+, or IHC 2+ with negative FISH

A newer category, “HER2-low,” describes tumors with IHC 1+ or 2+/FISH-negative. These are still considered HER2-negative for traditional classification, but they may now qualify for trastuzumab deruxtecan, an antibody-drug conjugate.

The pathology report also describes the histologic type (most TNBCs are invasive ductal carcinomas, often of “no special type”), the tumor grade (TNBC is usually grade 3 — poorly differentiated, fast-growing), and Ki-67, a marker of how rapidly the cells are dividing (typically high in TNBC).

Step 5: Staging

Once TNBC is confirmed, additional testing determines how far it has spread. Staging combines:

• Tumor size and local extent (T) — from imaging and, after surgery, from the surgical specimen
• Lymph node involvement (N) — assessed by ultrasound and biopsy of nearby nodes; during surgery, a sentinel lymph node biopsy or full axillary dissection provides definitive information
• Distant metastasis (M) — for higher-stage disease or symptoms suggesting spread, imaging such as CT of the chest/abdomen/pelvis, bone scan, or PET-CT is used; brain MRI is considered when neurologic symptoms are present, since TNBC has a relatively high rate of brain metastasis

These are combined into a stage from 0 to IV using the AJCC staging system, which now also incorporates tumor biology (grade, receptor status) alongside anatomic extent.

Step 6: Genetic testing

Because BRCA1 mutations are strongly associated with TNBC, germline genetic testing is recommended for all patients diagnosed with TNBC, regardless of age or family history. This is done with a blood or saliva sample. Results affect PARP inhibitor eligibility, surgical decisions (some patients opt for bilateral mastectomy), and screening recommendations for family members. Broader panels often test for BRCA2, PALB2, and other genes as well.

Last reviewed: 2026-05-30. This page is information only, not medical advice. Always discuss decisions about diagnosis or treatment with a qualified clinician.