Diagnostic Testing and Instrumentation
What the exam tests
Diagnostic testing questions ask which instrument measures which parameter, how to prepare the patient, and how to spot unreliable data before it reaches the chart.
Perimetry and imaging
Humphrey SITA fields: 24-2 covers the central 24°, 10-2 targets the macula, and 30-2 extends peripherally. Check reliability: fixation losses under 20%, false-positive and false-negative responses under 15%; review the glaucoma hemifield test, mean deviation, and pattern standard deviation. A field is not interpretable if the patient was sleepy or the lens was fogged. OCT quantifies retinal nerve fiber layer and ganglion cell thickness; accept only scans with adequate signal strength. Fundus photography documents; fluorescein angiography shows leakage and window defects; ICG images the choroidal circulation.
Biometry and corneal measurement
A-scan ultrasound measures axial length; B-scan is used when media opacity blocks the view. Repeat readings and check the average when values vary widely. Pachymetry measures central corneal thickness (normal about 540 µm); specular microscopy reports endothelial cell density (roughly 2,500 cells/mm²). Keratometry measures anterior corneal curvature; corneal topography maps irregular astigmatism such as keratoconus. Goldmann applanation requires fluorescein, cobalt-blue illumination, and a calibrated tonometer: IOP (mmHg) = 10 × force (g).
Worked example
The Goldmann dial reads 1.6 g. IOP = 1.6 × 10 = 16 mmHg. If the fluorescein semicircles are too thick, the examiner is applanating too much area and the force reading is too high, overestimating IOP; if the mires are too thin, IOP is underestimated. The correct endpoint is when the inner edges of the semicircles just touch.
Traps
Do not accept a field with high fixation losses; do not report a B-scan as an axial length; do not use an expired calibration sticker; do not compare pachymetry directly with IOP; do not perform Goldmann on an irregular cornea without noting artifact.