When to Validate Your Findings.

For Keratometry
A second person should confirm measurements prior to A-scan ultrasonography if:

• The corneal power is less than 40.0 diopters, or greater than 47.0 diopters.
• If there has been prior keratorefractive surgery. In this case the corneal power will need to be estimated by either the historical, or the contact lens method.
• The average corneal power difference between the two eyes is greater than 1.00 diopter.
• The patient cannot fixate, as seen with a mature cataract, or macular hole.
• The amount of corneal astigmatism by keratometry, or topography, correlates poorly with the amount of astigmatism on the most recent manifest refraction.
• The corneal diameter is less than 11.00 mm.
• There is any problem with patient cooperation, or understanding.

For Immersion A-scan Ultrasonography
A second person should re-measure both eyes if:

• The axial length is less than 22.00 mm, or greater than 25.00 mm in either eye.
• The axial length is greater than 26.0 mm, and there is a poor retinal spike, or wide variability in the readings. In this case, a B-scan should be scheduled to look for a (nasal or temporal) posterior staphyloma. At the same time, the axial length should be measured to the center of the macula by vector A-scan. If the center of the macula cannot be identified, the axial length should be measured 4.5 mm temporal to the void of the optic nerve head margin.
• There is a difference in axial length between the two eyes of greater than 0.33 mm that cannot be correlated with the patient's oldest refraction.
• Axial length measurements do not correlate with the patient's refractive error. In general, myopes should have eyes longer than 24.0 mm and hyperopes should have eyes shorter than 24.0 mm. Exceptions to this rule involve steep, or flat corneas. Be sure to use the oldest refractive data.
• There is difficulty obtaining correctly positioned, high, steeply rising echoes, or wide variability in individual axial length readings for either eye.
• There is any kind of problem with patient cooperation, or understanding.

For Intraocular Lens Power
A second person should repeat the axial length measurements, keratometry readings and re-run the IOL power calculations for both eyes if:

• The IOL power for emmetropia is greater than 3.00 diopters different than anticipated.
• There is a difference in IOL power of greater than 1.00 diopter between the two eyes.
• If the patient has had prior keratorefractive surgery and the calculated IOL power for standard phacoemulsification is less than +20.0 D or greater than +23.0 D.