Properly screening for protanopia, a color vision deficiency that limits the patient’s ability to discriminate red signal lights, is a crucial aspect of an ocular exam, considering the condition is problematic for certain occupations. However, not just any color vision test will do in this instance, new research suggests.

The study, published in Optometry and Vision Science, compared four clinical tests for color vision to diagnose protanopia from other color vision deficiencies. The researchers evaluated four tests—Hardy-Rand-Rittler (4th ed.), City University (3rd ed.), Ishihara and Mollon-Reffin—for their ability to discriminate between 18 protanopes and nine deuteranopes. The researchers noted that the Ishihara is the most common test used for occupational screening, while the others, although popular screening tools, aren’t necessarily fully validated for a full range of patients.

The investigation found the Ishihara test misdiagnosed all 18 protanopes as having a deutan deficiency. In contrast, the Hardy-Rand-Rittler and Mollon-Reffin tests correctly identified protan color vision deficiency in 100% of protanopes. No screening test was able to reliably diagnose protanopia on the basis of a strong protan color vision deficiency.

The Ishihara test is not suitable for screening for protanopia, and its failure to diagnose protanopes as having a protan color vision deficiency was far greater than what has been reported in previous studies, the researchers noted.

“The Hardy-Rand-Rittler and Mollon-Reffin are the most reliable tests for this purpose,” the researchers wrote in their paper.

 None of the screening tests were able to reliably differentiate dichromacy from strongly anomalous trichromacy.