diff --git a/tests/filter.c b/tests/filter.c
index 0138abc..4218212 100644
--- a/tests/filter.c
+++ b/tests/filter.c
@@ -195,49 +195,71 @@ void test_filter()
     struct cmt *cmt3;
     struct cmt *cmt4;
     struct cmt *cmt5;
+    struct cmt *cmt6;
     int flags = 0;
     char *fqname;
     char *label_key;
 
     cmt = generate_filter_test_data();
 
+    text = cmt_encode_text_create(cmt);
+    printf("[Not filtered] ====>\n%s\n", text);
+
+    cmt_encode_text_destroy(text);
+
     cmt2 = cmt_create();
     TEST_CHECK(cmt2 != NULL);
 
+    /* filter with fqname (SUBSTRING) */
+    fqname = "counter";
+    flags |= CMT_FILTER_SUBSTRING;
+    ret = cmt_filter(cmt2, cmt, fqname, NULL,
+                     NULL, NULL, flags);
+    TEST_CHECK(ret == 0);
+    /* check output (fqname) */
+    text = cmt_encode_text_create(cmt2);
+    printf("[substring matched with \"%s\" in fqname] ====>\n%s\n", fqname, text);
+
+    /* reset flags */
+    flags = 0;
+
+    cmt3 = cmt_create();
+    TEST_CHECK(cmt3 != NULL);
+
     /* filter with fqname (INCLUDE & PREFIX) */
     fqname = "spring";
     flags |= CMT_FILTER_PREFIX;
-    ret = cmt_filter(cmt2, cmt, fqname, NULL,
+    ret = cmt_filter(cmt3, cmt, fqname, NULL,
                      NULL, NULL, flags);
     TEST_CHECK(ret == 0);
 
     /* check output (fqname) */
-    text = cmt_encode_text_create(cmt2);
-    printf("====>\n%s\n", text);
+    text = cmt_encode_text_create(cmt3);
+    printf("[prefix matched with \"%s\" in fqname] ====>\n%s\n", fqname, text);
 
     cmt_encode_text_destroy(text);
 
     /* reset flags */
     flags = 0;
 
-    cmt3 = cmt_create();
-    TEST_CHECK(cmt3 != NULL);
+    cmt4 = cmt_create();
+    TEST_CHECK(cmt4 != NULL);
 
     /* filter with fqname (INCLUDE & SUBSTRING) */
     fqname = "load";
     flags |= CMT_FILTER_SUBSTRING;
-    ret = cmt_filter(cmt3, cmt, fqname, NULL,
+    ret = cmt_filter(cmt4, cmt, fqname, NULL,
                      NULL, NULL, flags);
     TEST_CHECK(ret == 0);
 
     /* check output (fqname) */
-    text = cmt_encode_text_create(cmt3);
-    printf("====>\n%s\n", text);
+    text = cmt_encode_text_create(cmt4);
+    printf("[substring matched with \"%s\" in fqname] ====>\n%s\n", fqname, text);
 
     cmt_encode_text_destroy(text);
 
-    cmt4 = cmt_create();
-    TEST_CHECK(cmt4 != NULL);
+    cmt5 = cmt_create();
+    TEST_CHECK(cmt5 != NULL);
 
     /* reset flags */
     flags = 0;
@@ -245,18 +267,18 @@ void test_filter()
     /* filter with label_key (EXCLUDE & PREFIX) */
     label_key = "host";
     flags |= CMT_FILTER_PREFIX;
-    ret = cmt_filter(cmt4, cmt, NULL, label_key,
+    ret = cmt_filter(cmt5, cmt, NULL, label_key,
                      NULL, NULL, flags);
     TEST_CHECK(ret == 0);
 
     /* check output (label_key) */
-    text = cmt_encode_text_create(cmt4);
-    printf("====>\n%s\n", text);
+    text = cmt_encode_text_create(cmt5);
+    printf("[prefix matched with \"%s\" in label key] ====>\n%s\n", label_key, text);
 
     cmt_encode_text_destroy(text);
 
-    cmt5 = cmt_create();
-    TEST_CHECK(cmt5 != NULL);
+    cmt6 = cmt_create();
+    TEST_CHECK(cmt6 != NULL);
 
     /* reset flags */
     flags = 0;
@@ -265,13 +287,13 @@ void test_filter()
     label_key = "label";
     flags |= CMT_FILTER_EXCLUDE;
     flags |= CMT_FILTER_SUBSTRING;
-    ret = cmt_filter(cmt5, cmt, NULL, label_key,
+    ret = cmt_filter(cmt6, cmt, NULL, label_key,
                      NULL, NULL, flags);
     TEST_CHECK(ret == 0);
 
     /* check output (label_key) */
-    text = cmt_encode_text_create(cmt5);
-    printf("====>\n%s\n", text);
+    text = cmt_encode_text_create(cmt6);
+    printf("[exclude with \"%s\" in label key] ====>\n%s\n", label_key, text);
 
     cmt_encode_text_destroy(text);
 
@@ -281,6 +303,7 @@ void test_filter()
     cmt_destroy(cmt3);
     cmt_destroy(cmt4);
     cmt_destroy(cmt5);
+    cmt_destroy(cmt6);
 }
 
 TEST_LIST = {