diff --git a/src/helpers/jsonataFunctions/transform.ts b/src/helpers/jsonataFunctions/transform.ts
index 0cd382d..fce23ae 100644
--- a/src/helpers/jsonataFunctions/transform.ts
+++ b/src/helpers/jsonataFunctions/transform.ts
@@ -93,7 +93,6 @@ export const transform = async (input, expression: string, extraBindings: Record
     bindings.parseCsv = parseCsv;
     bindings.v2parse = v2.v2parse;
     bindings.v2json = v2.v2json;
-    bindings.isNumeric = stringFuncs.isNumeric;
     bindings.capabilities = fhirFuncs.capabilities;
 
     const { aliases } = getCache();
@@ -128,6 +127,7 @@ export const transform = async (input, expression: string, extraBindings: Record
     expr.registerFunction('base64decode', stringFuncs.base64decode, '<s-:s>');
     expr.registerFunction('startsWith', stringFuncs.startsWith, '<s-s:a>');
     expr.registerFunction('endsWith', stringFuncs.endsWith, '<s-s:a>');
+    expr.registerFunction('isNumeric', stringFuncs.isNumeric, '<j-:a>');
 
     const res = await expr.evaluate(input, bindings);
     return res;
diff --git a/src/helpers/stringFunctions/stringFunctions.ts b/src/helpers/stringFunctions/stringFunctions.ts
index 7e0677b..fa6331d 100644
--- a/src/helpers/stringFunctions/stringFunctions.ts
+++ b/src/helpers/stringFunctions/stringFunctions.ts
@@ -77,15 +77,18 @@ export const matches = (str: string, regex: string) => {
   return fn.test(str);
 };
 
-export const isNumeric = (n: any) => {
-  if (typeof n === 'number') {
-    return true;
-  };
-  if (typeof n === 'string') {
-    return !isNaN(parseFloat(n)) && isFinite(parseFloat(n));
-  };
+export const isNumeric = (n: any): boolean | undefined => {
+  if (typeof n === 'number') return true;
+  if (typeof n === 'undefined') return undefined;
+
+  // RegEx taken form the FHIR spec for the decimal datatype: https://www.hl7.org/fhir/r4/datatypes.html
+  const isStrNumeric = (str: string) => /^-?(0|[1-9][0-9]*)(.[0-9]+)?([eE][+-]?[0-9]+)?$/.test(str);
+
+  if (typeof n === 'string') return isStrNumeric(n);
+
   if (Array.isArray(n) && n.length === 1) {
     return isNumeric(n[0]);
   };
+
   return false;
 };