S31.65AAOpen bite of abdominal wall, unspecified flank with penetration into peritoneal cavity, initial encounter

S31.65AA is the ICD-10-CM diagnosis code for open bite of abdominal wall, unspecified flank with penetration into peritoneal cavity, initial encounter. It belongs to category S31 (open wound of abdomen, lower back, pelvis and external genitals), block S30-S39 (injuries to the abdomen, lower back, lumbar spine, pelvis and external genitals) and chapter 19 (injury, poisoning and certain other consequences of external causes). It is billable, so it can be reported on its own for dates of service from October 1, 2026 through September 30, 2027.

The 7th character "A" means initial encounter: use it while the patient is receiving active treatment for the condition.

As a secondary diagnosis it is an MCC (major complication or comorbidity), which can move an inpatient stay into the highest-paying "with MCC" MS-DRG of its family, unless the principal diagnosis excludes it.

In MS-DRG v44.0, it is part of the grouping logic for DRG 393 (Other Digestive System Diagnoses with MCC, relative weight 1.5881), DRG 394 (Other Digestive System Diagnoses with CC, relative weight 0.9228), DRG 395 (Other Digestive System Diagnoses without CC/MCC, relative weight 0.6382), DRG 963 (Other Multiple Significant Trauma with MCC, relative weight 2.6363), DRG 964 (Other Multiple Significant Trauma with CC, relative weight 1.4972) and DRG 965 (Other Multiple Significant Trauma without CC/MCC, relative weight 1.0059), in MDC 06 (Diseases and Disorders of the Digestive System) and MDC 24 (Multiple Significant Trauma), as a principal or secondary diagnosis. The DRG a claim lands in depends on the principal diagnosis, procedures and any CC/MCC secondary diagnoses.

It is an unspecified code for which a more specific option (often laterality) exists, and the Medicare Code Editor flags it for review on inpatient claims.

It was added in FY2026, effective October 1, 2025.