ICD-10 code S31.157A: Open bite of left flank, initial encounter

Key takeaways

ICD-10 code S31.157A covers an open bite of the abdominal wall, left flank, without penetration into the peritoneal cavity, initial encounter.

It is a new FY2026 code, effective October 1, 2025, when the flank became its own site value.

S31.157 is not billable alone. Append A, D, or S so the claim carries a valid 7th character.

A bite that breaches the peritoneum goes to S31.657A, and S31.627A is a penetrating laceration with a foreign body.

The note must show a bite mechanism, the left flank, an intact peritoneum, and active treatment.

A dog bite across the left flank gets irrigated, closed, and sent home inside the hour. Then the claim bounces.

S31.157A is the ICD-10-CM code for an open bite of the abdominal wall, left flank, without penetration into the peritoneal cavity, initial encounter. It has only existed since October 1, 2025.

Before FY2026, a left flank bite had to be forced into a quadrant code. Old templates, cheat sheets, and claim scrubbers still point that way. Every descriptor below was checked against the CMS FY2026 code descriptions file rather than a secondary listing.

S31.157A covers a left flank bite that spared the peritoneum

S31.157A is a billable code for one situation. A bite broke the skin of the left flank and the peritoneal cavity stayed closed. The patient is also under active treatment at this visit.

You can confirm the descriptor in the CDC/NCHS ICD-10-CM lookup tool or in the FY2026 files on the CMS ICD-10 codes page. Both carry the same official wording.

Four facts sit inside the S31.157A descriptor

The official descriptor is Open bite of abdominal wall, left flank without penetration into peritoneal cavity, initial encounter. Four separate facts have to be in the record before you can use it.

1. Open bite. Teeth broke the skin surface. ICD-10-CM treats a bite as its own wound type, separate from lacerations, punctures, and abrasions.
2. Abdominal wall, left flank. The lateral abdominal wall on the left, between the lower ribs and the iliac crest. The flank is now its own site value, not part of a quadrant.
3. Without penetration into peritoneal cavity. The wound stops short of the peritoneum. Nothing inside the abdominal cavity is exposed.
4. Initial encounter. The patient is receiving active treatment for the injury at the visit you are coding.

Drop any one of those four and the code changes. A bite that did not break the skin is superficial and belongs to S30.871. A bite that reached the peritoneal cavity belongs to S31.657A.

The 7th character tells the payer which visit this is

S31.157 needs a 7th character to become billable. Three are valid, and each one marks a different phase of care.

The ICD-10-CM Official Guidelines tie “initial encounter” to active treatment, not to the calendar. A patient transferred to a second facility for ongoing wound management still gets the A character while active treatment continues.

Sequela coding reverses the order. List the condition itself first, such as the scar a dermatology practice is treating. S31.157S follows it to show what caused the scar.

That pattern runs through the whole injury chapter. A healing eyelid laceration takes D for the same reason and lands on S01.111D.

What counts as an open bite of the left flank?

An open bite is a wound where teeth have broken through the skin, leaving contaminated tissue and a track of variable depth. Dog bites dominate the presentation, followed by cat and human bites.

Flank bites tend to be shallower than they look because the lateral abdominal wall carries three muscle layers over the peritoneum.

That anatomy is exactly why the with-penetration and without-penetration split matters, and why the exam note has to answer it.

• Skin broken: a bite that only bruises or grazes is superficial, so it codes to S30.871 instead. Lingering symptoms from that kind of injury run to S30.9AXS.
• Left side: the record has to say left. “Flank bite” with no side goes to S31.15AA, unspecified flank.
• Flank, not quadrant: the flank sits lateral to the quadrants. If the note describes the left lower quadrant instead, use S31.154A.
• Peritoneum intact: the exam, wound exploration, or imaging has to support that the cavity was not entered.

Because this is the without-penetration code, it sits at the lower-acuity end of abdominal bite injuries.

Management is usually irrigation, debridement, a closure decision, tetanus and rabies review, and antibiotic cover. Clear documentation standards keep those findings retrievable when a payer asks.

S31.157A sits one branch away from a penetrating bite

Reading the chain top to bottom shows why the code means what it means. The with-penetration and without-penetration split happens at the fourth character, one level above the wound type.

One point is worth fixing in memory. Every code under S31.1 is a without-penetration code, and every code under S31.6 is a with-penetration code. S31.6 has no without-penetration branch, so a without-penetration bite can never be coded there.

The WHO ICD-10 browser shows the international skeleton of that hierarchy. Laterality, the flank site values, and the 7th character layer are all US clinical modification additions.

S31 also reaches wider than the abdominal wall. It covers the lower back, pelvis, and external genitals, which is why a neighbor like S31.33XA looks unrelated at first glance.

Sibling codes that get picked by mistake

Two axes generate the neighbors of S31.157A. Change the site and you stay inside S31.15. Change the wound type or the penetration status and you leave it.

Every code in the first table is an open bite of the abdominal wall without peritoneal penetration, initial encounter. Only the site differs.

The second table holds the left flank codes that are not open bites, plus the two with-penetration codes that get picked by mistake.

S31.657A is the decision that carries the most weight. It signals a breached peritoneal cavity, which changes triage, imaging, and the surgical pathway. Wound exploration findings or cross-sectional imaging should be in the note before a coder chooses between the two.

S31.627A is a different trap. The digits sit close to S31.157A, but that code is a penetrating laceration with a retained foreign body. Cross-check descriptors in the AAPC Codify ICD-10-CM lookup whenever two candidate codes differ by one digit.

Excludes notes decide what can share the claim

Four annotations in the tabular list govern S31.157A. They sit at different levels of the hierarchy, so all four are inherited.

• S31.15, Excludes1: superficial bite of abdominal wall (S30.871). A bite that leaves the skin intact never codes to S31.15, and the two can never be billed together.
• S31.1, Excludes2: open wound of abdominal wall with penetration into peritoneal cavity (S31.6-). Separate injuries can coexist, so both codes may appear on one claim.
• S31, Excludes1: traumatic amputation of part of abdomen, lower back and pelvis (S38.2-, S38.3). Tissue loss at that scale is an amputation, not an open wound.
• S31, Excludes2: open wound of hip (S71.00-S71.02) and open fracture of pelvis (S32.1-S32.9 with 7th character B). A flank bite plus a hip wound gets both codes.

Two more instructions attach further up. S31 carries a Code Also note for any associated spinal cord injury (S24.0, S24.1-, S34.0-, S34.1-) and for wound infection. Chapter S00-T88 tells you to add a Chapter 20 external cause code, plus Z18.- if a foreign body is retained.

Index terms that land on S31.157A, and ones that miss

The Alphabetic Index route is Bite(s) (animal) (human), then flank, then left, which lands on S31.157. Add the 7th character in the tabular list, never in the index.

• Maps here: “dog bite to left flank, skin broken, no peritoneal breach”, “human bite left lateral abdominal wall”, “cat bite left flank”.
• Inclusion term at subcategory level: S31.15 carries “Bite of abdominal wall NOS”, so an undocumented site still routes into this subcategory.
• Flank NOS: S31.15A carries “Open bite of abdominal wall of flank NOS without penetration into peritoneal cavity” for a flank bite with no side recorded.
• Does not map here: “superficial bite, skin intact” goes to S30.871, and “left lower quadrant bite” goes to S31.154A.

Six things the note must say before you bill it

S31.157A only holds up if the record supports every element of the descriptor. These are the six items an auditor looks for.

1. Bite mechanism. The note has to say bite. “Animal bite”, “dog bite”, or “human bite” works. “Open wound” or “laceration” does not support this code.
2. Skin breached. State that the skin surface was broken, which separates the injury from a superficial bite.
3. Left flank named. Record the side and the region. “Left flank” or “left lateral abdominal wall” both work.
4. Peritoneum intact. Record the exam, exploration, or imaging finding that rules out peritoneal penetration.
5. Encounter phase. Show whether the visit is active treatment, routine healing care, or management of a late effect.
6. External cause. Name the animal or circumstance so the Chapter 20 code can be assigned.

Bite wounds carry other decisions auditors expect to see recorded.

Tetanus status, rabies risk, and whether the wound was closed or left open all belong in the note. Practices that run vaccination programs, such as travel clinics, already log those dates as a matter of routine.

Digital intake forms keep the same prompts on screen during the encounter, which means fewer coder queries afterwards.

Five steps that reach S31.157A every time

Work through the descriptor in the same order every time. Each step rules a code family in or out.

Step 1: Confirm the skin was broken

An intact skin surface makes the injury superficial, and S30.871 applies. Only a wound that breaches the skin belongs in S31.

Step 2: Confirm the peritoneum was not entered

Without penetration keeps you in S31.1. Any documented peritoneal breach moves the claim to S31.6, which for a bite means S31.657A.

Step 3: Confirm the wound type is a bite

A bite mechanism takes you to S31.15. A laceration or puncture takes you to S31.11 through S31.14 instead, and a retained foreign body changes the choice again.

Step 4: Confirm the site is the left flank

The flank is now a distinct site value, so do not translate it into a quadrant. Left flank gives you S31.157, and an undocumented side gives you S31.15A.

Step 5: Add the 7th character and the external cause

Append A for active treatment, which completes S31.157A. Then add the Chapter 20 code for the mechanism, such as W54.0XXA for a dog bite or W55.01XA for a cat bite. Human bites split by intent, with W50.3XXA for accidental and Y04.1XXA for assault. Add Z20.3 when rabies exposure is suspected.

The CPT repair codes that pair with a flank bite

The flank counts as trunk for CPT repair coding, so the trunk ranges apply. Pick the repair level first, then the size band.

One range gets mis-picked often enough to name. 12020 and 12021 cover treatment of superficial wound dehiscence, by simple closure and with packing. Neither is a primary repair code.

For a fresh flank bite, simple repair runs from 12001 through 12007 and stops there. The 12011 to 12018 codes cover the face, ears, eyelids, nose, lips, and mucous membranes, so they never apply to the trunk.

Bite wounds are often left open rather than closed, which means no repair code applies at all. The visit then bills as evaluation and management. Debridement such as 11042, or a complex repair such as 13100, is added only when the note supports it.

Before you submit: The checks that stop a denial

The claim itself is short. S31.157A leads and the Chapter 20 external cause code follows. The procedure side carries either an evaluation and management level or a repair code.

Denials on a code this new are usually mechanical rather than clinical. Run these five checks before the claim leaves the practice.

• Date of service. It has to fall on or after October 1, 2025. An earlier visit uses the code set in force that day.
• 7th character. Active treatment takes A, and a later wound check takes D.
• Sequencing. S31.157A goes first and the external cause code follows, because a Chapter 20 code is never first-listed.
• Scrubber data. Confirm the code file is FY2026, so the flank values validate rather than reject.
• Note content. The record names the side, the region, the bite mechanism, and the intact peritoneum.

Two mistakes account for most of the rework. The first is a quadrant code picked out of habit, which pushes the claim through with the wrong site. The second is a missing external cause code, which some payers reject outright on injury claims.

How Pabau turns wound detail into a clean claim

Most flank bite queries come back to the same thing. The clinician knew the answer during the encounter, and the note never captured it. Side, depth, mechanism, and encounter phase then get reconstructed days later from a free-text paragraph.

Practice management software like Pabau moves those four fields into the template itself. Wound forms can require laterality, region, mechanism, and peritoneal findings before the note is signed, and photos attach to the same record.

Medical records management keeps that history in one place. A coder reviewing an S31.157A claim reads the original findings instead of chasing the clinician for them.

On the billing side, Pabau’s claims management software checks insurer details before a claim is submitted, which removes a common source of avoidable rejections. Pair that with compliance tools and audit trails, and the practice can show how a code was reached months later.

The outcome is fewer coder queries per injury encounter, a shorter path from visit to clean claim, and a record that survives an audit. Every Pabau subscription includes all of it.

Capture wound detail once, code it right

Pabau prompts your team for mechanism, laterality, depth, and encounter phase while the patient is still in the room. Coders then see every element of the S31.157A descriptor.

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Conclusion

S31.157A is narrow on purpose. It covers a bite that broke the skin of the left flank and stopped short of the peritoneal cavity. The visit also has to be active treatment. Move any one of those facts and a different code applies, whether that is S30.871, S31.154A, S31.15AA, or S31.657A.

The newness of the code is the practical risk. Scrubbers, cheat sheets, and wound templates all need updating before the first claim goes out. Get the template right and the coding takes care of itself. To see how Pabau supports injury coding from note to claim, book a demo.

Continue your research

Need the same breakdown for another open bite? S61.059A walks through an open bite of the thumb, including its 7th character options.

Wondering how S31 handles other wound types? S31.33XA covers a puncture wound in the same category, under the same 7th character rules.

Need the repair code that pairs with a trunk wound? 13101 sets out complex repair of the trunk from 2.6 cm to 7.5 cm.

Want a note layout that captures mechanism and site? Medical notes template gives you a structure an auditor can follow.

Not sure which NPI belongs on the claim? NPI types explains when to bill under the individual number and when to use the group.

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