19 Prefitting Evaluation Practice Questions & Answers

1. 1. The keratometer is calibrated to a fictitious corneal refractive index of 1.3375 rather than to the true refractive index of corneal tissue. Why is that value used?

   • A.It adjusts for the working distance between the instrument barrel and the patient's eye.
   • B.It offsets the minus power of the back corneal surface, so one front reading gives total power.✓ Answer
   • C.It converts reflected mire size into a radius without assuming any corneal shape at all.
   • D.It corrects for the tear film that pools over the corneal apex during the measurement.

   The keratometer measures only the reflection from the front corneal surface, but the cornea also has a negative-powered posterior surface. Using a reduced index of 1.3375 instead of the true tissue index builds that negative back-surface contribution into the reading, so a single anterior measurement approximates total corneal power.

   Source: Keratometry and the standard keratometric index of 1.3375 — Bennett & Henry, Clinical Manual of Contact Lenses, keratometry chapterReport a problem with this question

2. 2. A keratometer reads 45.00 D in one meridian. Using the standard keratometric constant of 337.5, what is the radius of curvature of that meridian?

   • A.8.04 mm
   • B.7.75 mm
   • C.7.06 mm
   • D.7.50 mm✓ Answer

   Dioptric power and radius are reciprocals linked by the keratometric constant, so radius in millimetres equals 337.5 divided by the power in dioptres. Here 337.5 divided by 45.00 gives 7.50 mm, and the same relationship shows why a higher dioptric reading always corresponds to a shorter, steeper radius.

   Source: Keratometric conversion r = 337.5/D — Bennett & Henry, Clinical Manual of Contact Lenses, keratometry chapterReport a problem with this question

3. 3. At prefitting, keratometry reads 42.50 D @ 180 and 44.25 D @ 090. What is the amount and type of this patient's corneal astigmatism?

   • A.1.75 D of oblique corneal astigmatism
   • B.1.75 D of against-the-rule corneal astigmatism
   • C.1.75 D of with-the-rule corneal astigmatism✓ Answer
   • D.0.88 D of with-the-rule corneal astigmatism

   Corneal astigmatism is simply the difference between the two keratometry readings, here 44.25 minus 42.50, which is 1.75 D. Because the steeper reading falls in the vertical meridian near 090, the astigmatism is with-the-rule; halving the difference would confuse corneal cylinder with a spherical equivalent.

   Source: Corneal astigmatism and axis orientation — Mannis & Zadnik, Contact Lenses in Ophthalmic Practice, keratometry and corneal topographyReport a problem with this question

4. 4. Which record most completely documents a prefitting evaluation, so that a later practitioner can judge whether the lens ordered was appropriate?

   • A.The history, the ocular measurements, the tear and slit lamp findings, and the reasoning.✓ Answer
   • B.The keratometry readings and the visible iris diameter, with no other examination data.
   • C.The lens order alone: manufacturer, base curve, diameter, power and dispensing date.
   • D.The spectacle refraction, the acuity obtained with it, and the patient's stated goals.

   A prefitting record has to support a clinical decision, not just a purchase, so it must show what was asked, what was measured, what the ocular surface looked like and why those findings led to that lens. A record containing only the order or only part of the measurements leaves the reasoning unverifiable and gives no baseline against which later changes can be compared.

   Source: Documentation of the prefitting evaluation — ABO-NCLE Contact Lens Registry Examination content outline, Prefitting: evaluation of dataReport a problem with this question

5. 5. During prefitting the keratometer mires look irregular and broken, and they clear briefly after the patient blinks several times. What is the most appropriate next step?

   • A.Record the mires as keratoconic and schedule a scleral lens fitting for the same day.
   • B.Order a rigid lens right away, since a rigid lens masks an irregular corneal surface.
   • C.Evaluate and treat the ocular surface, then repeat keratometry before choosing a lens.✓ Answer
   • D.Average the two most distorted readings and order a lens on that averaged base curve.

   Mires that distort and then clear transiently with blinking are being degraded by an unstable tear film rather than by the corneal shape itself, so the reading does not represent the true curvature. Ordering parameters from an unreliable measurement builds the error into the lens, and the surface problem would also compromise wear, so the surface is managed and the measurement repeated first.

   Source: Irregular keratometer mires and ocular surface disease — Bennett & Henry, Clinical Manual of Contact Lenses, prefitting evaluationReport a problem with this question

6. 6. A prefit tear break-up time is markedly shortened, with dry spots appearing almost immediately after a blink, and the lid margins show capped meibomian gland orifices. What does this most directly indicate for lens selection?

   • A.Keratometry will read flatter than the cornea is, so a steeper base curve is needed.
   • B.Tear volume is excessive, so a high-water lens will fit loosely and drop out.
   • C.The lipid layer is unstable, so the surface should be treated before a lens is chosen.✓ Answer
   • D.The mucin layer is absent, so a rigid material is the only one that can be tolerated.

   Capped meibomian orifices point to a deficient lipid layer, and it is the lipid layer that retards evaporation and holds the tear film together between blinks, which is why break-up occurs so early. A lens placed on a tear film that cannot survive one interblink period will dehydrate, deposit and feel dry, so the lid disease is addressed before any lens or material is committed to.

   Source: Tear break-up time and meibomian gland dysfunction — TFOS DEWS II Diagnostic Methodology reportReport a problem with this question

7. 7. A patient's horizontal visible iris diameter measures 11.8 mm. Applying the common soft lens rule that overall diameter should exceed the horizontal visible iris diameter by about 2.0 mm, which initial diameter should be selected?

   • A.12.2 mm
   • B.13.0 mm
   • C.15.5 mm
   • D.13.8 mm✓ Answer

   A soft lens must cover the cornea completely and land on scleral tissue all the way around, so its diameter is derived from the visible iris diameter plus a margin, here 11.8 plus 2.0, which gives 13.8 mm. A diameter chosen close to the iris diameter would leave the limbus exposed and let the edge ride onto the cornea, while a grossly oversized lens would bind on the conjunctiva.

   Source: Soft lens diameter selection from horizontal visible iris diameter — Contact Lens Society of America, The Contact Lens Manual, soft lens fittingReport a problem with this question

8. 8. A rigid gas permeable candidate has a narrow palpebral aperture, an upper lid that covers the superior limbus, and firm lid tension. Which initial design approach do these findings favor?

   • A.A truncated design, so the lower lid margin blocks all upward lens movement.
   • B.A lid-attachment design, with the lens riding superiorly beneath the upper lid.✓ Answer
   • C.An interpalpebral design, with a small lens centered between the lid margins.
   • D.A steep apical-clearance design, so the lens vaults the corneal apex entirely.

   When the upper lid already overlaps the superior limbus and has good tension, that lid can carry the lens and keep it positioned superiorly through the blink. Attaching the lens to the lid gives more stable centration, smoother blink-driven tear exchange and less edge sensation than letting a lens float between the lids in a narrow aperture.

   Source: Lid position, lid tension and rigid lens fitting philosophy — Bennett & Weissman, Clinical Contact Lens Practice, GP lens designReport a problem with this question

9. 9. Why is pupil diameter measured under dim illumination as part of the prefitting evaluation?

   • A.Because pupil size in dim light sets the overall diameter of the soft lens ordered.
   • B.Because a pupil larger than the optic zone in dim light produces flare and ghosting.✓ Answer
   • C.Because pupil size in dim light fixes the prism ballast a toric design will require.
   • D.Because a pupil smaller than the optic zone in dim light will decenter the lens.

   Light passing through the peripheral, non-optical portion of a lens is not focused with the rest of the image, so whenever the dark-adapted pupil extends beyond the optic zone the patient sees flare, halos and ghost images at night. Measuring the pupil in dim light identifies that mismatch before an optic zone size or a multifocal zone geometry is committed to.

   Source: Pupil size and optic zone diameter in lens design — Bennett & Weissman, Clinical Contact Lens Practice, GP lens designReport a problem with this question

10. 10. Relative to the flattest keratometry reading, how is the base curve of an initial soft lens selected, and why?

    • A.Steeper than steep K, because the edge must seal at the limbus to keep the lens centered.
    • B.Flatter than flat K, because the lens drapes and its sag must still exceed the cornea's.✓ Answer
    • C.Steeper than flat K, because a flexible lens must vault the apex in order to move.
    • D.Equal to flat K, because a soft lens aligns exactly along the flattest corneal meridian.

    A soft lens does not hold its own back-surface shape on the eye; it drapes over the cornea and extends well past the limbus, so what governs the fit is total sagittal depth rather than curve-to-curve alignment. Because the lens spans a much larger chord than the keratometer samples, its nominal base curve is numerically flatter than flat K while its sag still exceeds that of the cornea it covers.

    Source: Soft lens base curve selection relative to flat K — Bennett & Henry, Clinical Manual of Contact Lenses, soft lens fittingReport a problem with this question

11. 11. With the base curve held constant, ordering a soft lens in a larger overall diameter has what effect on the fit?

    • A.Sagittal depth is unchanged, since only the base curve determines the depth.
    • B.Sagittal depth decreases and the fit loosens, with more movement on the blink.
    • C.Sagittal depth increases and the fit tightens, with less movement on the blink.✓ Answer
    • D.Sagittal depth decreases but the fit tightens by covering more of the sclera.

    Sagittal depth is the height of the lens measured across its own chord, so extending that chord while keeping the same curvature necessarily deepens the lens. A deeper lens sits more snugly on the same eye, which is why diameter changes usually shift a soft fit more decisively than equivalent base curve changes do.

    Source: Sagittal depth in soft lens fitting — Efron, Contact Lens Practice, soft lens fitting chapterReport a problem with this question

12. 12. Two lenses are made of different materials. Lens A has the higher Dk, yet lens B delivers more oxygen to the cornea. What is the best explanation?

    • A.Oxygen reaching the cornea depends on Dk divided by thickness, and B is thinner.✓ Answer
    • B.Oxygen reaching the cornea depends on the wetting angle, and B wets more readily.
    • C.Oxygen reaching the cornea depends on water content, and B holds more water.
    • D.Oxygen reaching the cornea depends on overall diameter, and B is smaller in size.

    Dk describes only the material's intrinsic ability to pass oxygen, whereas the cornea experiences transmissibility, Dk divided by lens thickness. Because thickness sits in the denominator, a thick lens of high-Dk material can transmit less oxygen than a thin lens of lower-Dk material, which is why Dk/t and not Dk is compared against oxygen criteria such as those of Holden and Mertz.

    Source: Oxygen transmissibility (Dk/t) versus permeability (Dk); Holden and Mertz oxygen criteria — Efron, Contact Lens Practice, contact lens materialsReport a problem with this question

13. 13. In conventional hydrogels, oxygen permeability rises as water content rises. How does that relationship behave in silicone hydrogel materials?

    • A.Permeability is independent of composition and is governed by lens thickness alone.
    • B.Permeability rises as silicone content rises and water content falls, not with water.✓ Answer
    • C.Permeability falls as silicone content rises, because silicone impedes oxygen flow.
    • D.Permeability rises with water content, only more steeply than in conventional hydrogels.

    In a conventional hydrogel oxygen can only travel through the water phase, so more water means more oxygen. Silicone itself is a far better oxygen carrier than water, so in a silicone hydrogel the transport pathway is the silicone phase and permeability climbs as silicone is increased and water is reduced, exactly reversing the familiar hydrogel rule.

    Source: Silicone hydrogel oxygen permeability and water content — Efron, Contact Lens Practice, silicone hydrogel materialsReport a problem with this question

14. 14. Among hydrogel materials, which combination of water content and surface ionicity is associated with the greatest protein deposition and the greatest reactivity with care solutions?

    • A.High water content with an ionic surface.✓ Answer
    • B.Low water content with an ionic surface.
    • C.Low water content with a nonionic surface.
    • D.High water content with a nonionic surface.

    An ionic surface carries a negative charge that attracts positively charged tear proteins such as lysozyme, and a high water content gives a more open matrix into which those proteins can penetrate rather than merely resting on the surface. The same charged, open structure also makes the material more responsive to the pH, tonicity and preservatives of care solutions, which is why this group is the most deposit-prone of the hydrogel groupings.

    Source: FDA hydrogel contact lens material groups by water content and ionicity — U.S. Food and Drug Administration contact lens material groupingReport a problem with this question

15. 15. A patient's spectacle prescription is -2.00 -2.00 x 180, and keratometry reads 43.00 D @ 180 and 43.25 D @ 090. Based on these prefitting findings, which initial soft lens design is most appropriate?

    • A.A spherical rigid lens, because the tear lens will correct the cylinder present.
    • B.A back-surface soft toric, because the cylinder arises at the corneal surface.
    • C.A spherical soft lens, since a soft lens flexes enough to neutralize the cylinder.
    • D.A front-surface soft toric, because the cylinder is internal rather than corneal.✓ Answer

    The refraction contains 2.00 D of cylinder while the two keratometry readings differ by only 0.25 D, so roughly 1.75 D of the astigmatism is generated behind the cornea, mostly by the crystalline lens. Toricity ground on the back surface can only neutralize corneal toricity, so an internal cylinder of this size has to be carried on the front surface of a stabilized lens.

    Source: Residual astigmatism and selection of toric soft lens design — Mannis & Zadnik, Contact Lenses in Ophthalmic Practice, toric lens fittingReport a problem with this question

16. 16. A prospective wearer spends long days at a computer screen in a dry, air-conditioned office. Which mechanism most directly explains the dryness symptoms expected with lens wear?

    • A.Air conditioning raises humidity at the eye and swells the lens beyond parameters.
    • B.Blink rate rises, stripping the lipid layer and thinning the film over the lens.
    • C.Blink rate falls and blinks become incomplete, leaving lens and cornea exposed.✓ Answer
    • D.Screen glare constricts the pupil and reduces the tears held at the lid margin.

    Sustained visual attention on a screen reduces both how often a person blinks and how fully the lids close, and the blink is what resurfaces the lens with tears. Combined with low ambient humidity the lens surface dries between blinks, and the incompletely wiped inferior third of the lens is where desiccation and staining typically appear.

    Source: Blink rate, blink completeness and contact lens dryness — Efron, Contact Lens Practice, dry eye and contact lens wearReport a problem with this question

17. 17. A patient has ideal corneal measurements, a healthy tear film and an easily fitted prescription, but says a relative is pushing them into lenses and states they will not follow a cleaning routine. How should this affect the candidacy decision?

    • A.Order a daily-wear soft lens, since normal measurements make complications unlikely.
    • B.Select a rigid lens, since its smaller size makes handling and cleaning easier for anyone.
    • C.Dispense an extended-wear lens, since overnight use removes most of the handling burden.
    • D.Defer fitting until motivation and hygiene improve, since these predict success best.✓ Answer

    Anatomy determines which lens can be fitted, but motivation and compliance determine whether it will be worn safely and whether the patient stays in lenses at all. A wearer who does not want lenses and will not clean them faces a real infection risk, and no choice of parameters or modality substitutes for the behavior the modality still requires.

    Source: Patient motivation and compliance as predictors of contact lens success — Efron, Contact Lens Practice, patient selection and prefitting assessmentReport a problem with this question

18. 18. Which entry in a prefitting medication history most strongly suggests reduced aqueous tear production?

    • A.A topical antibiotic ointment used for a recent lid infection.
    • B.An oral calcium supplement taken for low bone density.
    • C.An inhaled bronchodilator used for exercise-induced asthma.
    • D.An oral antihistamine taken daily for seasonal allergy.✓ Answer

    Systemic antihistamines have anticholinergic activity, and the lacrimal gland depends on cholinergic stimulation for its secretion, so daily use lowers aqueous output and shortens tear stability. That matters at prefitting because the same drug class is common in patients who also have allergic conjunctivitis, so the lens is being placed on a surface that is both drier and already reactive.

    Source: Systemic medications with anticholinergic drying effects on tear production — TFOS DEWS II Iatrogenic Dry Eye reportReport a problem with this question

19. 19. At the prefitting slit lamp examination a patient shows a white stromal infiltrate with an overlying epithelial defect that stains, together with marked injection and pain. What is the appropriate action?

    • A.Withhold fitting and refer the patient for medical evaluation of a probable ulcer.✓ Answer
    • B.Fit a high-Dk silicone hydrogel, since added oxygen will speed healing of the defect.
    • C.Dispense a daily disposable, since a fresh lens each day lowers the infection risk.
    • D.Measure keratometry and tear film today and order lenses for a later dispensing visit.

    A stromal infiltrate with an overlying staining epithelial break, redness and pain describes a presumed infectious keratitis, which is an active corneal infection and an absolute contraindication to placing any lens on that eye. A lens would trap organisms against the wound and delay the antimicrobial treatment the eye needs, so fitting is withheld and the patient is referred for medical management.

    Source: Microbial keratitis as a contraindication to contact lens fitting and indication for referral — American Optometric Association, Care of the Contact Lens Patient (Optometric Clinical Practice Guideline)Report a problem with this question

Practice questions based on the ABO-NCLE Contact Lens Registry Exam content domains and standard contact lens references, together with the FTC Contact Lens Rule and FDA device classification. This site is not affiliated with or endorsed by ABO-NCLE. Contact lens fitting is regulated state by state — scope of practice, supervision and licensure requirements vary, so confirm your own state's rules and work under the prescriber's direction. Verify current exam requirements with ABO-NCLE before testing. About the ABO-NCLE exams →