diff --git a/integrationTests/Lists/addOrUpdateListItem.spec.ts b/integrationTests/Lists/addOrUpdateListItem.spec.ts
new file mode 100644
index 0000000..a489d62
--- /dev/null
+++ b/integrationTests/Lists/addOrUpdateListItem.spec.ts
@@ -0,0 +1,115 @@
+import { ListItemRequest } from './../../src';
+import { OnspringClient } from '../../src';
+import { expect } from 'chai';
+import { baseURL, apiKey } from '../mochaRootHooks';
+
+describe('addOrUpdateListItem', function () {
+  this.timeout(30000);
+  this.retries(3);
+
+  const newListItemIds: string[] = [];
+
+  // Delete the list items that were created during the test
+  after(async function () {
+    for (const id of newListItemIds) {
+      await deleteListItem(id);
+    }
+  });
+
+  it('should add a list item', async function () {
+    const client = new OnspringClient(baseURL, apiKey);
+
+    if (process.env.TEST_LIST_FIELD === undefined) {
+      expect.fail('TEST_LIST_FIELD is not defined');
+    }
+
+    if (process.env.TEST_LIST_ID === undefined) {
+      expect.fail('TEST_LIST_ID is not defined');
+    }
+
+    const request = new ListItemRequest(
+      parseInt(process.env.TEST_LIST_ID),
+      null,
+      `added_list_value_${new Date().getTime()}}`,
+      1,
+      '#000000'
+    );
+
+    const response = await client.addOrUpdateListItem(request);
+
+    expect(response.statusCode).to.equal(201);
+    expect(response.isSuccessful).to.be.true;
+    expect(response.message).to.equal('');
+    expect(response.data).to.not.be.null;
+
+    if (response.data != null) {
+      expect(response.data.id).to.not.be.null;
+      newListItemIds.push(response.data.id);
+    }
+  });
+
+  it('should update a list item', async function () {
+    const client = new OnspringClient(baseURL, apiKey);
+
+    if (process.env.TEST_LIST_FIELD === undefined) {
+      expect.fail('TEST_LIST_FIELD is not defined');
+    }
+
+    if (process.env.TEST_LIST_ID === undefined) {
+      expect.fail('TEST_LIST_ID is not defined');
+    }
+
+    const request = new ListItemRequest(
+      parseInt(process.env.TEST_LIST_ID),
+      null,
+      `added_list_value_${new Date().getTime()}}`,
+      null,
+      null
+    );
+
+    const response = await client.addOrUpdateListItem(request);
+    const listItemId = response.data?.id;
+
+    if (listItemId == null) {
+      expect.fail('new listItemId is null');
+    }
+
+    newListItemIds.push(listItemId);
+
+    const updateRequest = new ListItemRequest(
+      parseInt(process.env.TEST_LIST_ID),
+      listItemId,
+      `updated_list_value_${new Date().getTime()}}`,
+      1,
+      '#000000'
+    );
+
+    const updateResponse = await client.addOrUpdateListItem(updateRequest);
+
+    expect(updateResponse.statusCode).to.equal(200);
+    expect(updateResponse.isSuccessful).to.be.true;
+    expect(updateResponse.message).to.equal('');
+    expect(updateResponse.data).to.not.be.null;
+  });
+
+  // it('should return a 401 error when an invalid api key is used', async function () {});
+
+  // it('should return a 403 error when api key does not have access to the list', async function () {});
+
+  // it('should return a 404 error when the list does not exist', async function () {});
+
+  // it('should return a 404 error when the list item does not exist', async function () {});
+});
+
+async function deleteListItem(listItemIds: string): Promise<void> {
+  const client = new OnspringClient(baseURL, apiKey);
+
+  if (process.env.TEST_LIST_ID === undefined) {
+    return;
+  }
+
+  await client.deleteListItemById(
+    parseInt(process.env.TEST_LIST_ID),
+    listItemIds
+  );
+}
diff --git a/integrationTests/Lists/addOrUpdateListItemById.spec.ts b/integrationTests/Lists/addOrUpdateListItemById.spec.ts
deleted file mode 100644
index e69de29..0000000
diff --git a/src/models/ApiResponse.ts b/src/models/ApiResponse.ts
index 804a3d4..bba2439 100644
--- a/src/models/ApiResponse.ts
+++ b/src/models/ApiResponse.ts
@@ -219,17 +219,19 @@ export class ApiResponse<T> {
   }
 
   /**
-   * @method asCreatedWithIdResponseType - Converts the ApiResponse to an ApiResponse<CreatedWithIdResponse>.
-   * @returns {ApiResponse<CreatedWithIdResponse>} - An ApiResponse<CreatedWithIdResponse>.
+   * @method asCreatedWithIdResponseType - Converts the ApiResponse to an ApiResponse<CreatedWithIdResponse<T>>.
+   * @returns {ApiResponse<CreatedWithIdResponse<T>>} - An ApiResponse<CreatedWithIdResponse<T>>.
    */
-  public asCreatedWithIdResponseType(): ApiResponse<CreatedWithIdResponse> {
+  public asCreatedWithIdResponseType<T>(): ApiResponse<
+    CreatedWithIdResponse<T>
+  > {
     const apiResponse = this as ApiResponse<any>;
 
-    const createdWithIdResponse = new CreatedWithIdResponse(
+    const createdWithIdResponse = new CreatedWithIdResponse<T>(
       apiResponse.data.id
     );
 
-    return new ApiResponse<CreatedWithIdResponse>(
+    return new ApiResponse<CreatedWithIdResponse<T>>(
       apiResponse.statusCode,
       apiResponse.message,
       createdWithIdResponse
diff --git a/src/models/CreatedWithIdResponse.ts b/src/models/CreatedWithIdResponse.ts
index 3dbd2c4..9907e65 100644
--- a/src/models/CreatedWithIdResponse.ts
+++ b/src/models/CreatedWithIdResponse.ts
@@ -1,15 +1,15 @@
-export class CreatedWithIdResponse {
+export class CreatedWithIdResponse<T> {
   /**
-   * @property {number} id - The id of the created object.
+   * @property {T} id - The id of the created object.
    */
-  public id: number;
+  public id: T;
 
   /**
    * @constructor - Creates a new instance of the CreatedWithIdResponse class.
    * @param id - The id of the created object.
-   * @returns {CreatedWithIdResponse} - A new instance of the CreatedWithIdResponse class.
+   * @returns {CreatedWithIdResponse<T>} - A new instance of the CreatedWithIdResponse class.
    */
-  constructor(id: number) {
+  constructor(id: T) {
     this.id = id;
   }
 }
diff --git a/src/models/ListItemRequest.ts b/src/models/ListItemRequest.ts
index 5af2f65..eee1458 100644
--- a/src/models/ListItemRequest.ts
+++ b/src/models/ListItemRequest.ts
@@ -8,9 +8,9 @@ export class ListItemRequest {
   public listId: number;
 
   /**
-   * @property {string} id - The id for the list item.
+   * @property {string | null} id - The id for the list item.
    */
-  public id: string;
+  public id: string | null;
 
   /**
    * @property {string} name - The name of the list item.
@@ -18,30 +18,30 @@ export class ListItemRequest {
   public name: string;
 
   /**
-   * @property {number} numericValue - The numeric value of the list item.
+   * @property {number | null} numericValue - The numeric value of the list item.
    */
-  public numericValue: number;
+  public numericValue: number | null;
 
   /**
-   * @property {string} color - The color of the list item.
+   * @property {string | null} color - The color of the list item.
    */
-  public color: string;
+  public color: string | null;
 
   /**
    * @constructor - Creates a new instance of the ListItemRequest class.
    * @param {number} listId - The id for the list that the item belongs to.
-   * @param {string} id - The id for the list item.
+   * @param {string | null} id - The id for the list item.
    * @param {string} name - The name of the list item.
-   * @param {number} numericValue - The numeric value of the list item.
-   * @param {string} color - The color of the list item.
+   * @param {number | null} numericValue - The numeric value of the list item.
+   * @param {string | null} color - The color of the list item.
    * @returns {ListItemRequest} - A new instance of the ListItemRequest class.
    */
   constructor(
     listId: number,
-    id: string,
+    id: string | null,
     name: string,
-    numericValue: number,
-    color: string
+    numericValue: number | null,
+    color: string | null
   ) {
     this.listId = listId;
     this.id = id;
diff --git a/src/models/ListItemResponse.ts b/src/models/ListItemResponse.ts
index d68ecac..24dc238 100644
--- a/src/models/ListItemResponse.ts
+++ b/src/models/ListItemResponse.ts
@@ -1,15 +1,14 @@
 import { CreatedWithIdResponse } from './CreatedWithIdResponse';
-
 /**
  * @class ListItemResponse - Represents a respons when a list item is created or updated.
  */
-export class ListItemResponse extends CreatedWithIdResponse {
+export class ListItemResponse extends CreatedWithIdResponse<string> {
   /**
    * @constructor - Creates a new instance of ListItemResponse.
-   * @param {number} id - The id of the list item.
+   * @param {string} id - The id of the list item.
    * @returns {ListItemResponse} - A new instance of ListItemResponse.
    */
-  constructor(id: number) {
+  constructor(id: string) {
     super(id);
   }
 }
diff --git a/src/models/OnspringClient.ts b/src/models/OnspringClient.ts
index 2cdf228..b79696b 100644
--- a/src/models/OnspringClient.ts
+++ b/src/models/OnspringClient.ts
@@ -264,11 +264,11 @@ export class OnspringClient {
   /**
    * @method saveFile - Saves a file to a record in Onspring.
    * @param {SaveFileRequest} request - The request that will be used to save the file.
-   * @returns {Promise<ApiResponse<CreatedWithIdResponse>>} - A promise that resolves to an ApiResponse of type CreatedWithIdResponse.
+   * @returns {Promise<ApiResponse<CreatedWithIdResponse<number>>>} - A promise that resolves to an ApiResponse of type CreatedWithIdResponse.
    */
   public async saveFile(
     request: SaveFileRequest
-  ): Promise<ApiResponse<CreatedWithIdResponse>> {
+  ): Promise<ApiResponse<CreatedWithIdResponse<number>>> {
     const endpoint = EndpointFactory.getSaveFileEndpoint();
     const formData = request.asFormData();
     const apiResponse = await this.post<any>(endpoint, formData, {
@@ -279,7 +279,7 @@ export class OnspringClient {
       return apiResponse;
     }
 
-    return apiResponse.asCreatedWithIdResponseType();
+    return apiResponse.asCreatedWithIdResponseType<number>();
   }
 
   public async deleteFileById(
diff --git a/src/models/SaveRecordResponse.ts b/src/models/SaveRecordResponse.ts
index 0c3dcec..ad70380 100644
--- a/src/models/SaveRecordResponse.ts
+++ b/src/models/SaveRecordResponse.ts
@@ -1,6 +1,6 @@
 import { CreatedWithIdResponse } from './CreatedWithIdResponse';
 
-export class SaveRecordResponse extends CreatedWithIdResponse {
+export class SaveRecordResponse extends CreatedWithIdResponse<number> {
   public warnings: string[];
 
   constructor(id: number, warnings: string[] = []) {
diff --git a/tests/ApiResponse.spec.ts b/tests/ApiResponse.spec.ts
index 2a48785..1b4136f 100644
--- a/tests/ApiResponse.spec.ts
+++ b/tests/ApiResponse.spec.ts
@@ -780,7 +780,7 @@ describe('ApiResponse', function () {
       const createdWithIdResponse = apiResponse.asCreatedWithIdResponseType();
 
       expect(createdWithIdResponse).to.be.instanceOf(
-        ApiResponse<CreatedWithIdResponse>
+        ApiResponse<CreatedWithIdResponse<any>>
       );
       expect(createdWithIdResponse.data).to.be.instanceOf(
         CreatedWithIdResponse
diff --git a/tests/ListItemResponse.spec.ts b/tests/ListItemResponse.spec.ts
index 9de321a..67a68c8 100644
--- a/tests/ListItemResponse.spec.ts
+++ b/tests/ListItemResponse.spec.ts
@@ -12,7 +12,7 @@ describe('ListItemResponse', function () {
   });
 
   it('should have a constructor that takes an id and sets the id property', function () {
-    const id = 1;
+    const id = '3fa85f64-5717-4562-b3fc-2c963f66afa6';
     const listItemResponse = new ListItemResponse(id);
     expect(listItemResponse).to.have.property('id', id);
   });