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eye (version 0.1.0)

va_methods: VA conversion methods

Description

S3 methods for VA conversion

Usage

convertVA(x, to, ...)
# S3 method for quali
convertVA(x, to, snellnot, ...)
# S3 method for snellen
convertVA(x, to, snellnot, logmarstep, ...)
# S3 method for logmar
convertVA(x, to, snellnot, ...)
# S3 method for etdrs
convertVA(x, to, snellnot, ...)
# S3 method for default
convertVA(x, to, snellnot, ...)

Arguments

x

vector of visual acuities

to

to which VA class to convert

...

further arguments passed to methods

snellnot

which snellen notation. One of "ft", "m" or "dec"

logmarstep

how plus/minus entries are evaluated. Default to increase/decrease snellen fractions by lines. If TRUE, each snellen optotype will be considered equivalent to 0.02 logmar or 1 ETDRS letter (assuming 5 letters in a row in a chart)

Value

vector with visual acuity of class va. See also "VA classes"

Conversion

Although there seems to be no good statistical reason to convert back to Snellen, it is a very natural thing to eye specialists to think in Snellen. A conversion to snellen gives a good gauge of how the visual acuity for the patients are. However, back-conversion should not be considered an exact science and any attempt to use formulas will result in very weird Snellen values that have no correspondence to common charts. Therefore, Snellen matching the nearest ETDRS and logMAR value in the va_chart are used.

Further:

  • logMAR to ETDRS: logMAR rounded to the first digit and converted with the chart.

  • Snellen to logMAR: logMAR = -1 * log10(snellen_frac)

  • Snellen to ETDRS: ETDRS = 85 + 50 * log10(snellen_frac) Gregori et al..

  • ETDRS to logMAR: logMAR = -0.02 * etdrs + 1.7 Beck et al.

VA classes

convert_VA returns a vector of three classes:

  1. va

  2. One of snellen, logmar, etdrs or quali.

  3. Either of character (for Snellen and qualitative), numeric (for logMAR), or integer (for ETDRS).

Details

VA can be snellen feet/meter/decimal, logMAR, ETDRS, or "qualitative" (Counting fingers, etc.)

  • Snellen fractions need to be either form 6/x or 20/x

  • ETDRS must be between 0 and 100

  • logMAR must be between -0.3 and 3.0

  • Qualitative must be PL, LP, NLP, NPL, HM, CF (any case allowed)

Any element which is not recognized will be converted to NA

For other conversion rules see va

See Also

Other VA converter: snellen_steps, va_dissect(), va(), which_va()