Keratometry Test: Purpose, Test Details & Results

Purpose of Keratometry Test

Understanding the health and shape of the cornea is crucial for clear vision—and that's where the keratometry test comes in. Whether you're preparing for eye surgery, managing a condition like keratoconus, or simply need an accurate prescription for glasses or contact lenses, keratometry provides key measurements that guide safe and effective eye care. Let’s explore why this test is so important, who needs it, and what it helps doctors detect.

What Is Keratometry?

Keratometry is a non-invasive optical test that measures the curvature of the cornea—the transparent, dome-shaped front part of your eye. Since the cornea provides most of the eye’s focusing power, its shape directly affects how well you see. Keratometry specifically calculates the steepest and flattest meridians of the cornea, quantifying them in diopters (D), a unit of optical power.

Key Reasons for Performing Keratometry

• Diagnosis of Astigmatism and Irregularities: The test helps detect and quantify astigmatism (when the cornea is not perfectly spherical), which is a common cause of blurry or distorted vision. It’s also critical for identifying irregularities caused by corneal diseases like keratoconus, where the cornea thins and bulges outward 2, 6, 9.
• Surgical Planning: Before cataract surgery, keratometry ensures the correct intraocular lens (IOL) power is chosen. Even small errors in corneal measurement can lead to significant changes in postoperative vision quality 3, 5, 7.
• Monitoring Corneal Disease Progression: For patients with conditions such as keratoconus, repeated keratometry tests track changes in corneal shape, guiding treatment decisions and timing for interventions like crosslinking or surgery 6, 10.
• Corneal Transplantation and Eye Banking: In donor screening, keratometry evaluates the suitability of corneal tissue for transplantation, ensuring recipients get the best possible grafts 4.

Who Needs a Keratometry Test?

• People with blurry vision or astigmatism
• Candidates for cataract or refractive surgery
• Patients with diagnosed or suspected corneal diseases (e.g., keratoconus)
• Individuals undergoing contact lens fitting, especially with irregular corneas
• Recipients or donors in corneal transplantation programs

Why Accurate Corneal Measurement Matters

The shape of your cornea can subtly or dramatically impact your vision. Slight irregularities may just mean you need glasses, but more pronounced changes can lead to vision loss or complications after surgery if not properly accounted for in planning. That’s why keratometry is a cornerstone of modern eye care—it translates the complex curves of your cornea into actionable data for safer, more effective treatment.

Keratometry Test Details

The keratometry test is a fast, painless, and highly informative procedure, but the world of corneal measurement is more nuanced than it may appear. There are several types of keratometers and advanced imaging devices, each with distinct strengths. Understanding what happens during the test and how results can vary between devices is essential for interpreting the data and ensuring optimal outcomes.

How the Test Is Performed

• Preparation: You’ll be asked to sit comfortably and rest your chin and forehead on a support.
• Manual Keratometry: The examiner aligns a light source and observes reflections from the cornea, adjusting the instrument to measure the curve along different axes. This method is highly accurate but requires skill and cooperation from the patient 6.
• Automated Keratometry: Devices like the Topcon autokeratometer or Nidek handheld models quickly capture reflections and calculate curvature, reducing operator variability and speeding up the process 2, 3, 9.
• Imaging-Based Techniques:
  • Scheimpflug Cameras and OCT: These advanced systems create detailed, three-dimensional maps of the entire cornea, not just its central portion. They measure both the anterior and posterior corneal surfaces, allowing calculation of “total keratometry” or “true net power,” which can be especially useful in post-surgical or irregular corneas 2, 5, 7, 8.
  • Placido Disk Topography: Uses a series of concentric rings projected onto the cornea to map its shape in detail 6.
• Special Situations: In children or non-cooperative patients, handheld devices may be used, sometimes under general anesthesia. The method of eyelid separation (manual or with a speculum) does not significantly affect readings in such cases 3. In the context of eye banking, portable keratometers can assess donor eyes in situ before transplantation 4.

Types of Measurements

• Standard Keratometry (K): Measures the curvature of the anterior (front) surface of the cornea, usually in two meridians.
• Total Keratometry (TK): Incorporates both anterior and posterior corneal surfaces for a more complete picture 2, 7.
• Astigmatism Assessment: By comparing the steepest and flattest meridians, keratometry quantifies the degree and axis of astigmatism.
• Corneal Power (Diopters): The key output, used to calculate lens power and monitor disease progression.

Device Variability and Repeatability

• Device Agreement: In healthy eyes, different keratometry devices (automated, Scheimpflug, OCT) tend to agree closely, with differences less than 1 diopter. However, in eyes with corneal disease (e.g., keratoconus), agreement drops—especially as the condition worsens. Devices should not be used interchangeably in these cases 6, 8, 9.
• Repeatability: Most modern devices show excellent repeatability in normal eyes. In irregular or advanced keratoconus, measurement errors increase, highlighting the importance of using the same device for follow-up in these patients 6, 7, 8.

Limitations and Considerations

• Overestimation of Corneal Power: Conventional keratometry may overestimate total corneal power compared to advanced imaging-based calculations 2.
• Tear Film Quality: While tear film osmolarity can affect some eye measurements, keratometry repeatability is not significantly influenced by tear film osmolarity 8.
• Post-Surgical and Abnormal Corneas: In eyes with prior surgery or advanced disease, imaging-based approaches that measure total corneal power are preferable for accuracy 5, 7.

Keratometry Test Results & Follow-Up

Keratometry test results provide a window into the shape and health of your cornea—and they’re more than just numbers. These results guide diagnosis, treatment, surgical planning, and long-term monitoring. Understanding what the results mean, how they change over time, and what follow-up may be needed is crucial for effective eye care.

Interpreting the Numbers

• K Value (Diopters): The average healthy cornea measures between 42–45 diopters (D). Higher values indicate a steeper cornea; lower values, a flatter cornea. Significant deviations may indicate disease or prior surgical changes 2, 3, 7.
• Kmax (Maximum Keratometry): Especially relevant in keratoconus, where progressive steepening (rising Kmax) signals disease progression 10, 11, 12, 13.
• Astigmatism (Cylinder): Measured as the difference between steepest and flattest meridians. Significant astigmatism may require special corrective lenses or surgical planning 2, 9.

Monitoring and Disease Progression

• Keratoconus: Keratometry tracks changes in corneal curvature and helps diagnose keratoconus early. The progression of Kmax is a key marker—an increase of more than 1 diopter suggests worsening disease and may indicate need for intervention (e.g., corneal crosslinking) 6, 10, 11, 12, 13.
• Advanced Grading: Newer grading systems (e.g., ABCD) may detect progression earlier than Kmax alone, allowing for more timely treatment 10.
• Post-Surgical Changes: After procedures like LASIK or corneal crosslinking, keratometry documents stability or further change, guiding follow-up and additional interventions 13.

Follow-Up and Next Steps

• Stable Results: If keratometry readings are stable and within normal range, follow-up may simply involve routine eye exams 2, 3, 7.
• Progression Detected: Rising Kmax or new irregularity? Additional imaging, more frequent monitoring, or referral to a corneal specialist may be needed 6, 10, 11, 12, 13.
• Treatment Decisions: Significant progression may prompt recommendations for corneal crosslinking, specialty contact lenses, or surgical intervention. Outcomes can be dramatic—crosslinking can halt or even reverse steepening over years 11, 12, 13.
• Device Consistency: For those with corneal disease, it’s crucial that follow-up keratometry is done with the same device/method to ensure comparability and reliability 6, 9.

Limitations and Pitfalls

• Device Variability: In advanced keratoconus, different devices may yield results that aren’t interchangeable. Always interpret changes in the context of device consistency 6, 9.
• False Progression: Subtle measurement error or tear film changes can sometimes mimic true progression; confirm with repeat or adjunctive testing 8.

Conclusion

The keratometry test is a fundamental tool in modern eye care, shaping decisions from routine vision correction to complex surgical planning and disease management. Here’s a quick summary of what we’ve covered:

• Keratometry measures the cornea’s curvature, essential for diagnosing astigmatism, planning surgery, and tracking diseases like keratoconus 2, 6, 9, 10.
• Multiple devices and methods are available—manual, automated, and advanced imaging—each with unique strengths and best-use scenarios 2, 3, 6, 7, 8, 9.
• Interpreting keratometry results is vital for guiding follow-up, monitoring progression, and deciding on treatments such as corneal crosslinking or surgery 6, 10, 11, 12, 13.
• Device consistency and understanding limitations are key—especially in irregular or post-surgical corneas 6, 9.
• Early detection and regular monitoring via keratometry can dramatically improve outcomes for patients with corneal disorders.

By translating the complex curves of the cornea into actionable data, keratometry empowers patients and eye care professionals alike to make informed, life-changing decisions for better vision and quality of life.