Visual field quality control in the Ocular Hypertension Treatment Study (OHTS)

OBJECTIVE: To report the impact of visual field quality control (QC) procedures on the rates of visual field unreliability, test parameter errors, and visual field defects attributed to testing artifacts in the Ocular Hypertension Treatment Study (OHTS). METHODS: OHTS technicians were certified for perimetry and were required to submit 2 sets of visual fields that met study criteria before testing study participants. The OHTS Visual Field Reading Center (VFRC) evaluated 46,777 visual fields completed by 1618 OHTS participants between February 1994 and December 2003. Visual field QC errors, rates of unreliability, and defects attributed to testing artifacts were assessed. The OHTS QC system addressed 3 areas of clinic performance: (1) test parameter errors, (2) patient data errors, and (3) shipment errors. A visual field was classified as unreliable if any of the reliability indices exceeded the 33% limit. Clinical sites were immediately contacted by the VFRC via fax, e-mail, and/or phone and instructed on how to prevent further testing errors on fields with defects attributed to testing artifacts. MAIN OUTCOME MEASURES: QC errors (test parameter errors) and unreliability rates. RESULTS: A total of 2.4% (1136/ 46,777) of the visual fields were unreliable and 0.23% (107/46,777) had incorrect test parameters. Visual field defects attributed to testing artifacts occurred in approximately 1% (483/46,777) of the visual fields. CONCLUSIONS: Prompt transmission of visual fields to the VFRC for ongoing and intensive QC monitoring and rapid feedback to technicians helps to reduce the frequency of unreliable visual fields and incorrect testing parameters. Visual field defects attributed to testing artifacts were infrequent in the OHTS.