diff --git a/storage/remote/otlptranslator/prometheus/normalize_label.go b/storage/remote/otlptranslator/prometheus/normalize_label.go
index a112b9bbc..c22c76132 100644
--- a/storage/remote/otlptranslator/prometheus/normalize_label.go
+++ b/storage/remote/otlptranslator/prometheus/normalize_label.go
@@ -49,7 +49,7 @@ func NormalizeLabel(label string) string {
 
 // Return '_' for anything non-alphanumeric.
 func sanitizeRune(r rune) rune {
-	if unicode.IsLetter(r) || unicode.IsDigit(r) {
+	if unicode.IsLower(r) || unicode.IsUpper(r) || unicode.IsDigit(r) {
 		return r
 	}
 	return '_'
diff --git a/storage/remote/otlptranslator/prometheus/normalize_name.go b/storage/remote/otlptranslator/prometheus/normalize_name.go
index 0f472b80a..36b647f51 100644
--- a/storage/remote/otlptranslator/prometheus/normalize_name.go
+++ b/storage/remote/otlptranslator/prometheus/normalize_name.go
@@ -237,11 +237,13 @@ func removeSuffix(tokens []string, suffix string) []string {
 
 // Clean up specified string so it's Prometheus compliant
 func CleanUpString(s string) string {
-	return strings.Join(strings.FieldsFunc(s, func(r rune) bool { return !unicode.IsLetter(r) && !unicode.IsDigit(r) }), "_")
+	return strings.Join(strings.FieldsFunc(s, func(r rune) bool { return !unicode.IsUpper(r) && !unicode.IsLower(r) && !unicode.IsDigit(r) }), "_")
 }
 
 func RemovePromForbiddenRunes(s string) string {
-	return strings.Join(strings.FieldsFunc(s, func(r rune) bool { return !unicode.IsLetter(r) && !unicode.IsDigit(r) && r != '_' && r != ':' }), "_")
+	return strings.Join(strings.FieldsFunc(s, func(r rune) bool {
+		return !unicode.IsUpper(r) && !unicode.IsLower(r) && !unicode.IsDigit(r) && r != '_' && r != ':'
+	}), "_")
 }
 
 // Retrieve the Prometheus "basic" unit corresponding to the specified "basic" unit