diff --git a/sample/dynamic_test.go b/sample/dynamic_test.go
index 94823044f7..358d8d6f0e 100644
--- a/sample/dynamic_test.go
+++ b/sample/dynamic_test.go
@@ -19,7 +19,8 @@ func TestDynamicAddSampleRateKeyToTrace(t *testing.T) {
 
 	sampler := &DynamicSampler{
 		Config: &config.DynamicSamplerConfig{
-			FieldList: []string{"http.status_code"},
+			FieldList:  []string{"http.status_code", "root.service_name", "root.url"},
+			SampleRate: 1,
 		},
 		Logger:  &logger.NullLogger{},
 		Metrics: &metrics,
@@ -27,17 +28,32 @@ func TestDynamicAddSampleRateKeyToTrace(t *testing.T) {
 
 	trace := &types.Trace{}
 	for i := 0; i < spanCount; i++ {
+		if i == spanCount-1 {
+			trace.RootSpan = &types.Span{
+				Event: types.Event{
+					Data: map[string]interface{}{
+						"http.status_code": "200",
+						"service_name":     "test",
+					},
+				},
+			}
+		}
 		trace.AddSpan(&types.Span{
 			Event: types.Event{
 				Data: map[string]interface{}{
 					"http.status_code": "200",
+					"url":              "/test",
 				},
 			},
 		})
 	}
 	sampler.Start()
-	sampler.GetSampleRate(trace)
+	rate, keep, reason, key := sampler.GetSampleRate(trace)
 
 	spans := trace.GetSpans()
 	assert.Len(t, spans, spanCount, "should have the same number of spans as input")
+	assert.Equal(t, uint(1), rate)
+	assert.True(t, keep)
+	assert.Equal(t, "dynamic", reason)
+	assert.Equal(t, "200•,test,", key)
 }
diff --git a/sample/trace_key.go b/sample/trace_key.go
index da65fefa3f..7683397dd3 100644
--- a/sample/trace_key.go
+++ b/sample/trace_key.go
@@ -4,20 +4,34 @@ import (
 	"fmt"
 	"sort"
 	"strconv"
+	"strings"
 
 	"github.com/honeycombio/refinery/types"
 )
 
 type traceKey struct {
 	fields         []string
+	rootOnlyFields []string
 	useTraceLength bool
 }
 
 func newTraceKey(fields []string, useTraceLength bool) *traceKey {
 	// always put the field list in sorted order for easier comparison
 	sort.Strings(fields)
+	rootOnlyFields := make([]string, 0, len(fields)/2)
+	nonRootFields := make([]string, 0, len(fields)/2)
+	for _, field := range fields {
+		if strings.HasPrefix(field, RootPrefix) {
+			rootOnlyFields = append(rootOnlyFields, field[len(RootPrefix):])
+			continue
+		}
+
+		nonRootFields = append(nonRootFields, field)
+	}
+
 	return &traceKey{
-		fields:         fields,
+		fields:         nonRootFields,
+		rootOnlyFields: rootOnlyFields,
 		useTraceLength: useTraceLength,
 	}
 }
@@ -26,7 +40,7 @@ func newTraceKey(fields []string, useTraceLength bool) *traceKey {
 func (d *traceKey) build(trace *types.Trace) string {
 	// fieldCollector gets all values from the fields listed in the config, even
 	// if they happen multiple times.
-	fieldCollector := map[string][]string{}
+	fieldCollector := make(map[string][]string)
 
 	// for each field, for each span, get the value of that field
 	spans := trace.GetSpans()
@@ -54,6 +68,15 @@ func (d *traceKey) build(trace *types.Trace) string {
 		key += ","
 	}
 
+	if trace.RootSpan != nil {
+		for _, field := range d.rootOnlyFields {
+
+			if val, ok := trace.RootSpan.Data[field]; ok {
+				key += fmt.Sprintf("%v,", val)
+			}
+		}
+	}
+
 	if d.useTraceLength {
 		key += strconv.FormatInt(int64(len(spans)), 10)
 	}
diff --git a/sample/trace_key_test.go b/sample/trace_key_test.go
index e7a3bfec65..b9e29ba25c 100644
--- a/sample/trace_key_test.go
+++ b/sample/trace_key_test.go
@@ -117,8 +117,9 @@ func TestKeyGeneration(t *testing.T) {
 
 	assert.Equal(t, expected, generator.build(trace))
 
-	// now test that multiple values across spans in a different order are condensed the same
-	fields = []string{"http.status_code"}
+	// test field list with root prefix, only include the field from on the root span
+	// if it exists
+	fields = []string{"http.status_code", "root.service_name", "root.another_field"}
 	useTraceLength = true
 
 	generator = newTraceKey(fields, useTraceLength)
@@ -133,31 +134,24 @@ func TestKeyGeneration(t *testing.T) {
 		},
 	})
 
-	trace.AddSpan(&types.Span{
-		Event: types.Event{
-			Data: map[string]interface{}{
-				"http.status_code": 404,
-			},
-		},
-	})
-
 	trace.AddSpan(&types.Span{
 		Event: types.Event{
 			Data: map[string]interface{}{
 				"http.status_code": 200,
+				"service_name":     "another",
 			},
 		},
 	})
 
-	trace.AddSpan(&types.Span{
+	trace.RootSpan = &types.Span{
 		Event: types.Event{
 			Data: map[string]interface{}{
-				"http.status_code": 200,
+				"service_name": "test",
 			},
 		},
-	})
+	}
 
-	expected = "200•404•,4"
+	expected = "200•404•,test,2"
 
 	assert.Equal(t, expected, generator.build(trace))
 }