diff --git a/starlette/testclient.py b/starlette/testclient.py
index f4a960a3f..4de2e46a2 100644
--- a/starlette/testclient.py
+++ b/starlette/testclient.py
@@ -169,17 +169,16 @@ def send(
 
         request_complete = False
         response_started = False
-        response_complete = False
+        response_complete: anyio.Event
         raw_kwargs = {"body": io.BytesIO()}  # type: typing.Dict[str, typing.Any]
         template = None
         context = None
 
         async def receive() -> Message:
-            nonlocal request_complete, response_complete
+            nonlocal request_complete
 
             if request_complete:
-                while not response_complete:
-                    await anyio.sleep(0.0001)
+                await response_complete.wait()
                 return {"type": "http.disconnect"}
 
             body = request.body
@@ -203,7 +202,7 @@ async def receive() -> Message:
             return {"type": "http.request", "body": body_bytes}
 
         async def send(message: Message) -> None:
-            nonlocal raw_kwargs, response_started, response_complete, template, context
+            nonlocal raw_kwargs, response_started, template, context
 
             if message["type"] == "http.response.start":
                 assert (
@@ -225,7 +224,7 @@ async def send(message: Message) -> None:
                     response_started
                 ), 'Received "http.response.body" without "http.response.start".'
                 assert (
-                    not response_complete
+                    not response_complete.is_set()
                 ), 'Received "http.response.body" after response completed.'
                 body = message.get("body", b"")
                 more_body = message.get("more_body", False)
@@ -233,13 +232,14 @@ async def send(message: Message) -> None:
                     raw_kwargs["body"].write(body)
                 if not more_body:
                     raw_kwargs["body"].seek(0)
-                    response_complete = True
+                    response_complete.set()
             elif message["type"] == "http.response.template":
                 template = message["template"]
                 context = message["context"]
 
         try:
             with anyio.start_blocking_portal(**self.async_backend) as portal:
+                response_complete = portal.call(anyio.Event)
                 portal.call(self.app, scope, receive, send)
         except BaseException as exc:
             if self.raise_server_exceptions: