Sizeable inter- and intraobserver variability and substantial uncertainty may be observed for all tonometers, including Goldmann applanation tonometer (GAT), casting doubt on the validity of GAT as the default standard. Studies have been generally poorly reported.
Systematic review
The aim of systematic review was to compare the agreement of IOP readings of one or more tonometers in adults with the readings of GAT as the reference tonometer and to explore the factors affecting the agreement between tonometers including CCT and IOP level. Tonometry performed by any type of examiner including optometrists, ophthalmologists, nurses, technicians and patients was included. The primary outcome was the agreement between a tonometer and the reference standard. Secondary outcomes included inter- and intraobserver reliability for two observations. The quality of all included studies was assessed using a checklist.
A total of 102 studies reporting 130 comparisons involving 11 582 participants (15 525 eyes) were included. The studies took place in 26 countries in 1988-2010. The studies assessed the agreement of at least one tonometer with Goldmann Applanation Tonometer. Comparators were dynamic contour tonometer (DCT), non-contact tonometer (NCT), Ocular response analyser (ORA), Ocuton S, rebound tonometer (iCare), TonoPen, and transpalpebral tonometer. 99 studies had sufficient data for meta-analysis.
Studies were generally poorly reported. The agreement in IOP (95% limits) varied across tonometers, from 0.2 mmHg (–3.8 to 4.3 mmHg) for NCT to 2.7 mmHg (–4.1 to 9.6 mmHg) for Ocuton S. Sizeable inter- and intraobserver variability and substantial uncertainty was observed for all tonometers, including GAT, casting doubt on the validity of GAT as the default standard. Sensitivity analyses and subgroup analyses were undertaken to seek to identify sources of heterogeneity although with little light generated.
Due to magnitude of observed heterogeneity, it was not possible to undertake a subgroup analysis of CCT even on the basis of a crude dichotomy of the group for any of the studies. The exclusion of (based on reporting) lower-quality studies similarly did not provide clarity in this regard, although this perhaps reflects the substantial amount of non-reporting of key information in the studies.