## Introduction

What are EUCAST rules? The European Committee on Antimicrobial Susceptibility Testing (EUCAST) states on their website:

EUCAST expert rules are a tabulated collection of expert knowledge on intrinsic resistances, exceptional resistance phenotypes and interpretive rules that may be applied to antimicrobial susceptibility testing in order to reduce errors and make appropriate recommendations for reporting particular resistances.

In Europe, a lot of medical microbiological laboratories already apply these rules (Brown et al., 2015). Our package features their latest insights on intrinsic resistance and unusual phenotypes (v3.3, 2021).

Moreover, the eucast_rules() function we use for this purpose can also apply additional rules, like forcing ampicillin = R in isolates when amoxicillin/clavulanic acid = R.

## Examples

These rules can be used to discard impossible bug-drug combinations in your data. For example, Klebsiella produces beta-lactamase that prevents ampicillin (or amoxicillin) from working against it. In other words, practically every strain of Klebsiella is resistant to ampicillin.

Sometimes, laboratory data can still contain such strains with ampicillin being susceptible to ampicillin. This could be because an antibiogram is available before an identification is available, and the antibiogram is then not re-interpreted based on the identification (namely, Klebsiella). EUCAST expert rules solve this, that can be applied using eucast_rules():

oops <- data.frame(
mo = c(
"Klebsiella",
"Escherichia"
),
ampicillin = "S"
)
oops
#>            mo ampicillin
#> 1  Klebsiella          S
#> 2 Escherichia          S

eucast_rules(oops, info = FALSE)
#>            mo ampicillin
#> 1  Klebsiella          R
#> 2 Escherichia          S

A more convenient function is mo_is_intrinsic_resistant() that uses the same guideline, but allows to check for one or more specific microorganisms or antibiotics:

mo_is_intrinsic_resistant(
c("Klebsiella", "Escherichia"),
"ampicillin"
)
#> [1]  TRUE FALSE

mo_is_intrinsic_resistant(
"Klebsiella",
c("ampicillin", "kanamycin")
)
#> [1]  TRUE FALSE

EUCAST rules can not only be used for correction, they can also be used for filling in known resistance and susceptibility based on results of other antimicrobials drugs. This process is called interpretive reading, is basically a form of imputation, and is part of the eucast_rules() function as well:

data <- data.frame(
mo = c(
"Staphylococcus aureus",
"Enterococcus faecalis",
"Escherichia coli",
"Klebsiella pneumoniae",
"Pseudomonas aeruginosa"
),
VAN = "-", # Vancomycin
AMX = "-", # Amoxicillin
COL = "-", # Colistin
CAZ = "-", # Ceftazidime
CXM = "-", # Cefuroxime
PEN = "S", # Benzylenicillin
FOX = "S", # Cefoxitin
stringsAsFactors = FALSE
)
data
mo VAN AMX COL CAZ CXM PEN FOX
Staphylococcus aureus - - - - - S S
Enterococcus faecalis - - - - - S S
Escherichia coli - - - - - S S
Klebsiella pneumoniae - - - - - S S
Pseudomonas aeruginosa - - - - - S S
eucast_rules(data)
mo VAN AMX COL CAZ CXM PEN FOX
Staphylococcus aureus - S R S S S S
Enterococcus faecalis - - R R R S R
Escherichia coli R - - - - R S
Klebsiella pneumoniae R R - - - R S
Pseudomonas aeruginosa R R - - R R R