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Consider patient response variability when assessing MPOD
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Two comparison studies for heterochromic flicker photometry instruments demonstrate reliability, but clinicians should consider patient variability when using this type of measurement as an indicator for age-related macular degeneration.
Elizabeth Wyles, OD, FAAO, and Robert Donati, PhD, explained in their abstract at the American Academy of Optometry annual meeting that previous studies have suggested that reduced levels of macular pigment may increase risk for developing AMD. These studies were the impetus to evaluate the reliability and variability of the clinically available instruments to measure macular pigment.
An earlier study they presented at ARVO 2012 involved 28 patients with and without signs of early age-related macular degeneration older than 50. Macular pigment optical density (MPOD) measurements were taken using MacuScope (Marco, Jacksonville, Fla.) and QuantifEye (ZeaVision, Chesterfield, Mo.).
The overall mean MPOD for the cohort was 0.341±0.149 for MacuScope and 0.317±0.191 for QuantifEye. The mean standard deviation for repeated measurements on individual subjects was 0.107 and 0.058 for MacuScope and QuantifEye, respectively. When considering individual subject variability, there was a significant difference between the two instruments (P < .0006).
“[In the first study] we took patients from our clinic that would be more representative of patients who would be taking the test. We found that there was variability within the two instruments, and that’s what led us to [the second] study,” Wyles said in an interview with Primary Care Optometry News. “Let’s get the perfect patient – if possible – and would that change the results?”
For the second study, presented at the academy meeting, 17 young adults were recruited from the Illinois Eye Institute/Illinois College of Optometry. MPOD was measured using the two instruments.
“We started looking at the young, normal, healthy population and we’re seeing similar differences [between the instruments] with them, then it seems to be more instrument-driven variability than patient-driven variability,” Donati said. “If you look at the mean of all the patients, the machines look very similar, so it doesn’t look like there’s much difference, but when you start looking at the standard deviations from individual repeated measurements, you start seeing significant differences between machines.”
The mean MPOD was 0.315±0.131 for the MacuScope (n = 29) and 0.370±0.159 for the QuantifEye (n = 29). The standard deviations from individual repeated measurements for the MPOD were 0.069±0.083 (n = 29) for the MacuScope and 0.040±0.041 (n = 32) for the QuantifEye. There was no significant difference between the MPOD means when comparing the two instruments. However, when considering individual subject variability, differences between machines were not statistically significant, but there was a definite difference.
“The clinician must take into account individual patient variability if using MPOD as an indicator for AMD risk and/or clinical care,” the authors wrote.
“In general terms, based on our findings, we would expect to find variability in both populations on both instruments, with less variability in the younger adults, Wyles said. “However, neither instrument and neither population resulted in a variability less than what we established as acceptable for our study.” – by Cheryl DiPietro
References:
Wyles E, Donati RJ. A clinical comparison of the MacuScope and QuantifEye macular pigment densitometers. Paper presented at: ARVO 2012; May 2012; Fort Lauderdale, FL.
Wyles E, Donati RJ. A comparison of the MacuScope and QuantifEye macular pigment densitometers. Poster presented at: Academy 2012; October 2012; Phoenix, AZ.
For more information:
Robert Donati, PhD, can be reached at the Illinois College of Optometry; (312) 949-7136; rdonati@ico.edu.
Disclosure: Wyles and Donati report no relevant financial disclosures.