2026 ICD-10-CM Diagnosis Code R18.0Malignant ascites

ICD-10-CM Codes›R00–R99›R10-R19›R18

ICD-10-CM R18.0
CMSSource: CMS FY 2026 ICD-10-CM dataset · Effective Oct 1, 2025 – Sep 30, 2026

R18.0 is a billable ICD-10-CM diagnosis code for malignant ascites. It is valid on HIPAA claims for fiscal year 2026 (October 1, 2025 through September 30, 2026) and groups to MS-DRG 947 through 948. The code is not accepted as a principal diagnosis by the Medicare Code Editor. As a symptom code, it should not be used as a principal diagnosis once a related definitive diagnosis has been established. Coders also document this condition as ascitic fluid examination abnormal. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Abdominal pain and other digestive/abdomen signs and symptoms and Conditions due to neoplasm or the treatment of neoplasm.

Code Identity

ICD-10-CM Code
R18.0
Billable Status
Yes — Valid for Submission
Code Describes
Malignant ascites
Short Description
Malignant ascites
Same as the full description in the CMS dataset.
Parent Code
R18
Ascites

Code Classification

ChapterR00–R99Symptoms, signs and abnormal clinical and laboratory findings, not elsewhere classified
SectionR10-R19Symptoms and signs involving the digestive system and abdomen
CategoryR18Ascites
This CodeR18.0Malignant ascites

Code EditsBilling

Medicare Code Editor checks that affect claim validity for R18.0.

Unacceptable principal diagnosis
There are selected codes that describe a circumstance which influences an individual's health status but not a current illness or injury, or codes that are not specific manifestations but may be due to an underlying cause. These codes are considered unacceptable as a principal diagnosis.

Approximate SynonymsGuidance

Alternate terms and clinical phrases that map to this code.

  • Ascitic fluid examination abnormal
  • Ascitic fluid: malignant cells
  • Malignant ascites
  • Peritoneal fluid abnormal

Tabular List NotesGuidance

Coding notes and annotation back-references applicable to this code.

Code First

  • malignancy, such as:
  • malignant neoplasm of ovary C56
  • secondary malignant neoplasm of retroperitoneum and peritoneum C78.6

Index to Diseases and InjuriesGuidance

Alphabetical index entries that point to this code.

External Cause of Injuries IndexGuidance

References for this code in the External Cause of Injuries Index.

    • Ascites(abdominal)
      • malignant

Clinical ClassificationClinical

AHRQ’s CCSR groups this code into broader clinical categories.

CCSR SYM006
Abdominal pain and other digestive/abdomen signs and symptoms
Default principal diagnosis: inpatient No · outpatient No
CCSR NEO074
Conditions due to neoplasm or the treatment of neoplasm
Default principal diagnosis: inpatient No · outpatient Yes

Clinical InformationClinical

  • Ascites

    accumulation or retention of free fluid within the peritoneal cavity.

  • Carcinoma, Ehrlich Tumor

    a transplantable, poorly differentiated malignant tumor which appeared originally as a spontaneous breast carcinoma in a mouse. it grows in both solid and ascitic forms.

  • Chylous Ascites

    presence of milky lymph (chyle) in the peritoneal cavity, with or without infection.

  • Mucin-4

    a transmembrane mucin that is found in a broad variety of epithelial tissue. mucin-4 may play a role in regulating cellular adhesion and in cell surface signaling from the erbb-2 receptor protein-tyrosine kinase. mucin-4 is a heterodimer of alpha and beta chains. the alpha and beta chains result from the proteolytic cleavage of a precursor protein.

  • Peritoneovenous Shunt

    an operation for the continuous emptying of ascitic fluid into the venous system. fluid removal is based on intraperitoneal and intrathoracic superior vena cava pressure differentials and is performed via a pressure-sensitive one-way valve connected to a tube traversing the subcutaneous tissue of the chest wall to the neck where it enters the internal jugular vein and terminates in the superior vena cava. it is used in the treatment of intractable ascites.

  • Pseudomyxoma Peritonei

    a peritoneal adenocarcinoma characterized by build-up of mucus in the peritoneal cavity. mucus secreting cells may attach to the peritoneal lining and continue to secrete mucus. the majority of cases represent tumor spread from a primary low-grade mucinous neoplasm of the appendix (nci thesaurus).

  • Receptor, ErbB-2

    a cell surface protein-tyrosine kinase receptor that is overexpressed in a variety of adenocarcinomas. it has extensive homology to and heterodimerizes with the egf receptor, the erbb-3 receptor, and the erbb-4 receptor. activation of the erbb-2 receptor occurs through heterodimer formation with a ligand-bound erbb receptor family member.

Patient EducationClinical

Liver Diseases

Your liver is the largest organ inside your body. It helps your body digest food, store energy, and remove poisons.

Read the full article at MedlinePlus

Courtesy of MedlinePlus, a service of the U.S. National Library of Medicine.

Convert R18.0 to ICD-9-CMHistory

The closest ICD-9-CM equivalents under the General Equivalence Mappings.

ICD-9-CM
789.51 Malignant ascites
Exact Match The mapping is direct, with no qualifiers.

Code HistoryHistory

FY 2016AddedAdded to the ICD-10-CM code setEffective October 1, 2015, the first year of ICD-10-CM.
FY 2017–2025No changes
FY 2026CurrentCurrent code set, no changesEffective October 1, 2025 through September 30, 2026.

Questions About R18.0Overview

Is R18.0 (Ascites) a billable code?

Yes. This is a billable ICD-10-CM code, specific enough to report malignant ascites on HIPAA-covered claims from October 1, 2025 through September 30, 2026.

What MS-DRG does R18.0 group to?

On inpatient claims, malignant ascites maps to MS-DRG 947, 948, with relative weights from 0.8005 to 1.2694 depending on complications. Higher weights mean higher Medicare reimbursement.

Can R18.0 be a principal diagnosis?

No. The Medicare Code Editor rejects this code as a principal diagnosis because malignant ascites describes a circumstance that influences health status rather than a current illness. Report it as a secondary diagnosis.

What is the ICD-9 equivalent of R18.0?

Under the General Equivalence Mappings, malignant ascites converts to ICD-9-CM 789.51 (malignant ascites). The mapping is a direct match.

Footnotes

[1] Chronic - a chronic condition code indicates a condition lasting 12 months or longer and its effect on the patient based on one or both of the following criteria:

  • The condition results in the need for ongoing intervention with medical products,treatment, services, and special equipment
  • The condition places limitations on self-care, independent living, and social interactions.