When reviewing a medical report, unfamiliar terms like CK19 can lead to questions. Cytokeratin 19 (CK19) is a protein found within cells throughout the body. Its detection in medical tests helps understand cellular health and identify certain conditions. This protein serves as a marker for pathologists and doctors to gather information about cells and tissues, particularly when evaluating potential diseases.
The Role of CK19 in Healthy Cells
Cytokeratin 19 is a component of the cytoskeleton, which acts as the internal scaffolding within a cell. This network of protein filaments provides structural integrity, helping cells maintain their shape and organize internal components. CK19 is a type I cytokeratin, an acidic protein that typically pairs with basic cytokeratins to form intermediate filaments.
The protein is normally found in simple and glandular epithelial cells, which line the surfaces of organs and ducts throughout the body. Examples include cells lining the gastrointestinal tract, bile ducts, and certain glands. CK19 is also expressed in the periderm, a temporary superficial layer that covers the developing epidermis during fetal development. The presence of CK19 in these healthy cells is a normal and necessary aspect of their function, contributing to their structural stability.
CK19 as a Diagnostic Biomarker
Medical professionals test for CK19 because it functions as a biomarker, an indicator of a biological state or condition. In pathology, CK19 can provide clues about the nature and origin of abnormal cells. The primary method to detect CK19 in tissue samples is immunohistochemistry (IHC), which uses specific antibodies to make the protein visible under a microscope.
CK19 is a diagnostic tool because many cancer cells retain cytokeratins from their original parent cells. When cancer spreads to other areas of the body, such as lymph nodes, these metastatic cancer cells often continue to express CK19. This allows pathologists to identify the likely origin of the cancer cells, even when they appear in a distant location. For example, if cancer cells are found in a lymph node and stain positive for CK19, it helps confirm an epithelial origin, guiding further diagnostic steps.
Interpreting CK19 Test Results
When a pathology report indicates “positive expression” or “CK19-positive,” it means the CK19 protein was detected in the cells being examined. This detection is often visualized as cytoplasmic staining under a microscope. A positive CK19 result is not, by itself, a cancer diagnosis, as CK19 is normally present in many healthy epithelial cells. The significance of a positive result depends on the clinical context.
CK19-positive cells are particularly meaningful when found in an area where they should not normally be, such as a lymph node that is free of primary epithelial tissue. Within a tumor, the presence and pattern of CK19 expression help pathologists classify the tumor type and determine its likely origin. Pathologists use CK19 as part of a panel of several biomarkers, which may include other cytokeratins like CK7 and CK20, or specific markers like TTF-1 (Thyroid Transcription Factor-1). This comprehensive approach creates a more complete diagnostic picture, distinguishing between different tumor types and guiding patient management.
Cancers Associated with CK19 Expression
CK19 expression is useful in classifying several types of cancer, aiding pathologists in refining diagnoses. Its presence or absence, often in combination with other markers, helps differentiate between various tumor subtypes.
Thyroid Cancer
CK19 is a marker used in diagnosing papillary thyroid carcinoma (PTC), the most common type of thyroid cancer. CK19 is often strongly and diffusely expressed in PTC, including its follicular variant. While some benign thyroid lesions may show focal or weak CK19 staining, a diffuse and strong staining pattern in a thyroid lesion with a follicular growth pattern, especially in older patients, raises the suspicion of malignancy.
Breast Cancer
CK19 is widely expressed in breast carcinomas, with over 90% of cases showing positivity, particularly in luminal subtypes like ductal carcinoma. Invasive ductal carcinoma (IDC) typically shows positive CK19 staining. Conversely, invasive lobular carcinoma (ILC), the second most common type of breast cancer, is often associated with a loss of E-cadherin expression and may be CK19-negative. This difference in expression can help distinguish between these two major breast cancer subtypes.
Liver and Pancreatic Cancers
CK19 is valuable in diagnosing cancers originating from the bile ducts, known as cholangiocarcinoma. It is also positive in most adenocarcinomas found in the gastrointestinal tract and pancreas. In the liver, CK19 can help differentiate cholangiocarcinoma from hepatocellular carcinoma (HCC), as CK19 is not expressed in most hepatocytes but is often diffusely positive in cholangiocarcinoma. For pancreatic neuroendocrine tumors, CK19 positivity is associated with aggressive behavior and a poorer outcome, regardless of tumor size or lymphovascular invasion.
Lung Cancer
CK19 is a marker for non-small cell lung cancers (NSCLC), including adenocarcinoma and squamous cell carcinoma. It is detected in a high percentage of these tumors, with over 90% expression in lung adenocarcinomas and squamous cell carcinomas. While CK19 is widely expressed in various malignancies, its consistent presence in NSCLC makes it a useful indicator, especially when evaluating lymph node metastases where CK19 expression from the primary tumor is often maintained.