Study questions accuracy of CCT-adjusted IOP measurements

This retrospective case series assessed the usefulness of central corneal thickness (CCT)-based IOP correction formulae for stratified CCT groups (thin, intermediate and thick), with IOP measured with the Pascal dynamic contour tonometer (PDCT) as the reference standard. The study found that adjusting IOP using CCT-based formulae resulted in poorer agreement with PDCT IOP when compared with unadjusted Goldmann applanation tonometry (GAT) IOP. This suggests that although CCT may be useful in population analyses, CCT-based correction formulae should not be applied to individuals.

This appears to be a very significant study as it will cause a reassessment of the usefulness of CCT-adjusted IOP, which has become a routine part of decision-making regarding treatment of patients with elevated IOP. Most significant is the finding that GAT IOP is more accurate than CCT-corrected IOP. It should also be noted that PDCT IOP was always higher than the IOP as measured by all other tonometers, including CCT-corrected measurements.

The authors compared the GAT, Ocular Response Analyzer and CCT-adjusted IOP measurements of 289 consecutive patients attending a specialty glaucoma practice – a mixture of normal subjects and subjects with confirmed glaucoma.

There was a 26 to 39 percent risk of making an erroneous IOP adjustment of at least 20 percent for all ranges of CCT. The risk of error was greatest in patients with thicker corneas, in which increased IOP may be interpreted as a GAT measurement artifact. With the exception of corneal compensated IOP, all of the IOP measurement and adjustment methods increased the risk of creating clinically significant error compared with PDCT.

The authors note that the poor usefulness of CCT-based measurement in patients with thicker corneas also was observed in a group with confirmed glaucomatous optic neuropathy that was analyzed separately. As in the overall group, CCT-based correction tended to underestimate IOP in the thickest tercile. This underestimation could lead to delayed treatment, the risk of which may outweigh that of patients being misclassified with ocular hypertension and inappropriately administered long-term therapy.

Despite the study’s limitations, the authors say its results show there is a risk of creating clinically significant error after adjustment of GAT IOP with CCT-based correction formulae. This error could expose patients with thicker corneas to the risk of delayed diagnosis or undertreatment.