Accuracy of portable and traditional non-contact tonometers in measuring different intraocular pressure levels

Abstract:

AIM:To evaluate the accuracy of a portable non-contact tonometer(PNCT)compared with a traditional non-contact tonometer(NCT)in measuring intraocular pressure(IOP)at different levels.

METHODS:Patients who presented to Ningbo Eye Hospital between January 2025 and January 2026 were retrospectively included. All patients underwent measurements using PNCT, NCT, and Goldmann applanation tonometry(GAT). Based on the GAT measurements, the patients were categorized into a low IOP group(IOP≤16 mmHg), a moderate IOP group(16 mmHgRESULTS: A total of 94 patients(184 eyes)were included in this study, comprising 52 males and 42 females with a mean age of 48.6±16.4 y. The median IOP values measured by GAT, PNCT, and NCT were 19.3(15.5, 24.0), 19.3(16.0, 24.1), and 19.0(15.0, 26.0)mmHg, respectively(χ²=3.77, P>0.05). Spearman correlation analysis revealed that the IOP values measured by both PNCT and NCT were positively correlated with those measured by GAT(r_s=0.86 and 0.84, respectively, P<0.05). Intraclass correlation coefficient(ICC)analysis demonstrated an ICC of 0.89 between PNCT and GAT(P<0.05), and 0.88 between NCT and GAT(P<0.05), with an overall ICC of 0.88 among the three instruments(P<0.05). Bland-Altman analysis indicated a mean bias of 0.1 mmHg \〖95% limits of agreement(LoA): -7.3 to 7.6 mmHg\〗 between PNCT and GAT, and a mean bias of 0.1 mmHg(95%LoA: -8.4 to 8.6 mmHg)between NCT and GAT. Based on the GAT measurements, the eyes were categorized into a low IOP group(60 eyes, 32.6%, IOP≤16 mmHg), a moderate IOP group(63 eyes, 34.2%, 16 mmHgLoA: -4.4 to 7.4 mmHg)for PNCT and 0.5 mmHg(95%LoA: -5.6 to 6.6 mmHg)for NCT. In the moderate IOP group, the mean bias was -0.2 mmHg(95%LoA: -5.4 to 5.1 mmHg)for PNCT and -0.1 mmHg(95%LoA: -8.6 to 8.4 mmHg)for NCT. In the high IOP group, the mean bias was -0.8 mmHg(95%LoA: -10.6 to 8.9 mmHg)for PNCT and -0.1 mmHg(95%LoA: -10.5 to 10.4 mmHg)for NCT. Furthermore, 175 eyes underwent central corneal thickness(CCT)measurements, with a mean CCT of 553.3±35.9(450-660)μm. The IOP measurements from GAT(r_s=0.31), PNCT(r_s=0.39), and NCT(r_s=0.40)all exhibited positive correlations with CCT(P<0.05). Notably, the measurement difference between PNCT and GAT was not correlated with CCT(r_s=0.08, P>0.05), whereas the difference between NCT and GAT showed a positive correlation with CCT(r_s=0.23, P<0.05).

CONCLUSION:The IOP measurements obtained by PNCT, NCT, and GAT demonstrated strong correlation and good agreement, with PNCT being less influenced by CCT. However, as IOP levels increased, the measurement errors of both PNCT and NCT also increased. Therefore, PNCT and NCT readings should only serve as references and cannot replace GAT.