Difference in intraocular pressure measurements between non-contact tonometry and Goldmann applanation tonometry and the role of central corneal thickness in affecting glaucoma referrals.

BACKGROUND: Patients at glaucoma risk are commonly identified by optometrists and subsequently referred to glaucoma specialists. Optometrists mainly use non-contact tonometry (NCT) for intraocular pressure (IOP) measurement.

AIMS: To investigate the role of differences in IOP measurement between NCT and Goldmann applanation tonometry (GAT) and the effect of central corneal thickness (CCT) on these differences in optometrist referrals METHODS: Details of the initial clinical visit of patients referred with IOP > 21 mmHg in either eye as measured by NCT to a consultant glaucoma specialist were retrospectively reviewed. Demographic and referral data, IOP, CCT, and glaucoma diagnosis were obtained. The main outcome measure was the IOP measurement differences between NCT and GAT.

RESULTS: Of the 98 patients referred, only 23% had IOP > 21 mmHg when measured by GAT. NCT (Nidek NT400, Reichert Puff, Pulsair Easy Eye) measured the IOP greater than GAT by a mean of 5.8 mmHg (NCT 24.1 ± 3.5, GAT 18.3 ± 3.0). The effect of CCT on IOP measurement was less for GAT (R2 0.034, p = 0.067) than for NCT (R2 0.088, p = 0.003). The NCT/GAT IOP differences increased with increasing CCT (R2 0.166, p < 0.0001). The NCT/GAT differences decreased with patient age (R2 0.048, p = 0.03). Patients were classified as normal 67% (66/98), ocular hypertension 11% (11/98), glaucoma suspect 14% (14/98), and glaucoma 7% (7/98).

CONCLUSIONS: The difference in IOP measurement between NCT and GAT leads to a possible increase in glaucoma referrals, particularly in patients with thicker corneas. Repeat IOP using GAT and CCT measurement would help in triaging referrals.