Diagnosis & Your Test Results: The Path to Confirmation

Confirming a diagnosis of acromegaly involves more than just a single blood test. Because your hormones naturally fluctuate throughout the day, doctors use “dynamic” testing to see how your body responds to specific triggers ^[1]. This process usually follows a three-step path: screening, confirmation, and imaging.

Step 1: The IGF-1 Screening

The first step is measuring Insulin-like Growth Factor 1 (IGF-1) ^[1]. Unlike Growth Hormone (GH), which pulses up and down, IGF-1 levels stay relatively stable in your blood, making it the most reliable “snapshot” of your average hormone activity ^[2].

• Age and Sex Matter: Your “normal” IGF-1 level depends heavily on how old you are and your sex ^[1]. A level that is healthy for a teenager would be dangerously high for a 50-year-old. When you look at your lab report, ensure your result is being compared to the reference range for your specific age group ^[1].
• Interferences: Conditions like liver disease, kidney failure, or poorly controlled diabetes can sometimes cause a “false low” IGF-1, even if acromegaly is active ^[2][3].

Step 2: The Sugar Water Test (OGTT)

To confirm the diagnosis, doctors use the Oral Glucose Tolerance Test (OGTT), often called the “gold standard” ^[4]. During this test, you will drink a standardized, highly concentrated glucose (sugar) drink. The medical team will then draw your blood multiple times over a period of about two hours to see how your hormones react. In a healthy body, drinking a high-sugar solution causes the pituitary gland to stop making Growth Hormone ^[4]. In acromegaly, the tumor ignores the sugar and continues to pump out GH ^[1].

• The GH Nadir: This is the lowest level your Growth Hormone reaches during the test.
• The Threshold: Using modern, highly sensitive tests, a GH nadir of less than 0.4 µg/L is generally considered a normal result that “rules out” active acromegaly ^[4][5]. If your GH stays above this level, it confirms the tumor is producing hormone autonomously ^[1].

Step 3: Pituitary MRI & The Knosp Grade

Once the blood tests confirm the hormone excess, a gadolinium-enhanced MRI is used to find the tumor ^[6]. Gadolinium is a contrast dye that helps the tumor stand out from the normal pituitary gland ^[7].

Tumor Size

Your report will likely classify the tumor based on its size:

• Microadenoma: Less than 10 millimeters (mm) ^[6].
• Macroadenoma: 10 millimeters or larger ^[6]. Macroadenomas are more common in acromegaly and may require more complex treatment if they press on nearby structures like the optic nerves ^[8].

Understanding the Knosp Grade

The Knosp scale is a grading system (0 to 4) that tells your surgeon how far the tumor has spread sideways into a nearby area called the cavernous sinus ^[9].

• Grades 0, 1, and 2: These are considered “non-invasive.” The tumor is contained or only slightly touching nearby structures. These usually have higher rates of successful surgical removal ^[10][9].
• Grades 3 and 4: These indicate the tumor has “invaded” or wrapped around the internal carotid artery ^[9]. While still treatable, these higher grades are more challenging for surgeons to remove completely ^[11][12].

Knowing these specific terms—GH nadir, macroadenoma, and Knosp grade—will help you have a much more detailed conversation with your endocrinologist and surgeon about your treatment plan.

Read next: Standard of Care Treatment: Surgery & Medical Therapies