diff --git a/DESCRIPTION b/DESCRIPTION
index bb22c74f5..0f537f4d7 100644
--- a/DESCRIPTION
+++ b/DESCRIPTION
@@ -1,6 +1,6 @@
 Package: AMR
-Version: 2.1.1.9279
-Date: 2025-05-16
+Version: 2.1.1.9281
+Date: 2025-05-20
 Title: Antimicrobial Resistance Data Analysis
 Description: Functions to simplify and standardise antimicrobial resistance (AMR)
   data analysis and to work with microbial and antimicrobial properties by
diff --git a/NEWS.md b/NEWS.md
index 36037fade..fc03da7e7 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -1,4 +1,4 @@
-# AMR 2.1.1.9279
+# AMR 2.1.1.9281
 
 *(this beta version will eventually become v3.0. We're happy to reach a new major milestone soon, which will be all about the new One Health support! Install this beta using [the instructions here](https://amr-for-r.org/#get-this-package).)*
 
diff --git a/R/sir.R b/R/sir.R
index fdc671111..9cd03b156 100755
--- a/R/sir.R
+++ b/R/sir.R
@@ -445,9 +445,9 @@ is_sir_eligible <- function(x, threshold = 0.05) {
   %in% class(x))) {
     # no transformation needed
     return(FALSE)
-  } else if (!all(is.na(x)) && all(toupper(x) %in% c("S", "SDD", "I", "R", "NI", NA))) {
+  } else if (!all(is.na(x)) && all(x %in% c("S", "SDD", "I", "R", "NI", NA, "s", "sdd", "i", "r", "ni"))) {
     return(TRUE)
-  } else if (!all(is.na(x)) && !any(c("S", "SDD", "I", "R", "NI") %in% gsub("([SIR])\\1+", "\\1", gsub("[^A-Z]", "", toupper(x), perl = TRUE), perl = TRUE), na.rm = TRUE)) {
+  } else if (!all(is.na(x)) && !any(c("S", "SDD", "I", "R", "NI") %in% gsub("([SIR])\\1+", "\\1", gsub("[^A-Z]", "", toupper(unique(x[1:10000])), perl = TRUE), perl = TRUE), na.rm = TRUE)) {
     return(FALSE)
   } else {
     x <- x[!is.na(x) & !is.null(x) & !x %in% c("", "-", "NULL")]