2026 ICD-10-CM Diagnosis Code R18.0Malignant ascites
ICD-10-CM Codes›R00–R99›R10-R19›R18
- Billable — Valid for Submission
- CC — Complication or Comorbidity
- Risk Adjusts — HCC 17
- Not a Principal Diagnosis
- Chronic Condition
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. As a secondary diagnosis, it counts as a complication or comorbidity (CC) and moves an inpatient stay to a higher severity level within its MS-DRG family. It does not count, however, when the principal diagnosis is one of 57 closely related codes. 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.
For Medicare Advantage risk adjustment, R18.0 maps to CMS-HCC Category 17 (Cancer Metastatic to Lung, Liver, Brain, and Other Organs; Acute Myeloid Leukemia Except Promyelocytic) under the V28 model, adding a risk factor of about 4.209 for a community, non-dual, aged beneficiary in payment year 2026.
Code Identity
Code Classification
Code EditsBilling
Medicare Code Editor checks that affect claim validity for R18.0.
Medicare Risk Adjustment (HCC)Billing
R18.0 maps to a payment category in the CMS-HCC model used to risk-adjust Medicare Advantage payments. Weights are the published community factors for payment year 2026.
Source: CMS Payment Year 2026 risk adjustment mappings and model software. Weights are relative factors, not dollar amounts; a beneficiary's total RAF also includes demographics and interactions. Browse all CMS-HCC categories.
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
Certain conditions have both an underlying etiology and multiple body system manifestations due to the underlying etiology. For such conditions, the ICD-10-CM has a coding convention that requires the underlying condition be sequenced first followed by the manifestation. Wherever such a combination exists, there is a "use additional code" note at the etiology code, and a "code first" note at the manifestation code. These instructional notes indicate the proper sequencing order of the codes, etiology followed by manifestation.
Index to Diseases and InjuriesGuidance
Alphabetical index entries that point to this code.
Clinical ClassificationClinical
AHRQ’s CCSR groups this code into broader clinical categories.
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.
Code HistoryHistory
Questions About R18.0Overview
What is the ICD-10 code for malignant ascites?
The ICD-10-CM code for malignant ascites is R18.0 (sometimes written as R180). It is billable on HIPAA-covered claims from October 1, 2025 through September 30, 2026.
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.
Is R18.0 a CC or MCC?
CMS lists R18.0 as a CC (complication or comorbidity) for FY 2026. Reported as a secondary diagnosis, it moves the inpatient stay to a higher-weighted DRG within its severity family. It does not count when the principal diagnosis is one of the 57 closely related codes in its exclusion list.
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 HCC is R18.0?
R18.0 (malignant ascites) maps to CMS-HCC Category 17 (Cancer Metastatic to Lung, Liver, Brain, and Other Organs; Acute Myeloid Leukemia Except Promyelocytic), commonly written as HCC 17, in the CMS-HCC V28 model used for Medicare Advantage risk adjustment in payment year 2026. It does not map to any RxHCC in the Part D prescription drug model.
Does R18.0 risk-adjust for Medicare Advantage payment?
Yes. When documented and reported on a Medicare Advantage encounter, R18.0 adds a risk adjustment factor of about 4.209 to the beneficiary's RAF score for a community, non-dual, aged enrollee (published V28 weights range from 1.952 to 4.235 depending on the payment segment). HCC 17 sits at the top of its hierarchy, so no other condition category supersedes it. See the full factor table on the HCC 17 category page.
