E87.1: Hypo-osmolality and hyponatremia

The ICD-10-CM code for hyponatremia, the single billable code covering low serum sodium and reduced plasma osmolality within category E87.

Billable

FY2027

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At a glance

Code Family

E87: Other disorders of fluid, electrolyte and acid-base balance.

Condition

Hyponatremia

Billable

Yes, a single billable code with no child codes.

HCC Mapping (v28)

None. E87.1 does not map to any CMS-HCC v28 category.

Family Specificity

E87.1 is the sole code for hyponatremia and hypo-osmolality within category E87. E87.1 carries no severity axis. Hyponatremia documented as SIADH is excluded at E87.1 and coded to E22.2.

Effective Date

Valid in the FY2026 code set and unchanged in the FY2027 update.

Which E87 code do I assign?

The E87 family splits on which electrolyte or acid-base disturbance the note documents rather than on severity or etiology, with E87.1 as the single code for low serum sodium. Assignment turns on the direction of the sodium abnormality and on whether a category Excludes1 note redirects the encounter to a more specific code.

When documentation says…

Code

Hyponatremia, serum sodium below reference range

E87.1

Hypo-osmolality and hyponatremia. The note must record hyponatremia as a confirmed diagnosis that was evaluated, monitored or treated, not a laboratory value standing alone.

Elevated serum sodium, hypernatremia

E87.0

Hyperosmolality and hypernatremia. This is the directional opposite of E87.1. Assign it when the note documents high rather than low serum sodium.

Low serum potassium

E87.6

Hypokalemia. Potassium disturbances take their own codes within category E87. Do not assign E87.1 unless the note separately documents low sodium.

Electrolyte imbalance associated with hyperemesis gravidarum

O21.1

Hyperemesis gravidarum with metabolic disturbance. An Excludes1 note at category E87 bars E87.1 for the same electrolyte imbalance in this obstetric context.

Electrolyte imbalance following ectopic or molar pregnancy

O08.5

Metabolic disorders following an ectopic and molar pregnancy. An Excludes1 note at category E87 directs the encounter here rather than to E87.1.

Electrolyte disturbance due to diabetes insipidus

E23.2

Diabetes insipidus. An Excludes1 note at category E87 bars reporting E87.1 together with E23.2 for the same condition.

Hyponatremia from furosemide taken as prescribed

E87.1

Hypo-osmolality and hyponatremia, sequenced first as the nature of the adverse effect. The adverse effect code T50.1X5A follows in second position. A T-code is not acceptable as the principal diagnosis in an adverse effect scenario.

Tabular notes

EXCLUDES1

Category E87 carries Excludes1 notes that are inherited by E87.1: diabetes insipidus (E23.2), electrolyte imbalance associated with hyperemesis gravidarum (O21.1), electrolyte imbalance following ectopic or molar pregnancy (O08.5), familial periodic paralysis (G72.3), and metabolic acidemia in newborn, unspecified (P19.9). E87.1 carries its own Excludes1 note for syndrome of inappropriate secretion of antidiuretic hormone (E22.2). An Excludes1 note means the two codes are not reported together for the same condition. Guideline I.A.12.a allows both codes when the conditions are unrelated. When one of these excluded conditions is the documented diagnosis, assign that code rather than E87.1.

FY2027: EFFECTIVE 1 OCTOBER 2026

The FY2027 codes addenda add and delete no codes in category E87. The only FY2027 tabular change in the category rewords the Code first note at E87.22, chronic metabolic acidosis. E87.1 is unchanged in the FY2027 update. No codes were added to or deleted from category E87 in FY2026. No Excludes note changes were identified for E87.1 in the FY2026 tabular addenda.

Operational & financial impact

Why specificity matters beyond the chart: risk adjustment, DRG assignment and denial exposure all follow from which code is selected.

CMS-HCC v28 (PY2024+)

E87.1 does not map to any Hierarchical Condition Category under CMS-HCC Model Version 28.

CMS-HCC v28 is fully operative at 100 percent weight for Medicare Advantage payment year 2026.

Reporting E87.1 does not contribute to a beneficiary risk score.

V24 TO V28 TRANSITION

Electrolyte and fluid balance disorders, including hyponatremia, are not risk adjusting conditions under CMS-HCC v24 or under v28. The move between model versions does not change the status of E87.1.

MEDICARE AND COMMERCIAL PAYERS

There is no National Coverage Determination specific to E87.1. The code supports medical necessity for electrolyte monitoring, serum osmolality testing, sodium replacement therapy, and nephrology or endocrinology referral. Coverage for associated services is governed by MAC-specific Local Coverage Determinations.

WHERE SPECIFICITY IS LOST

E87.1 does not identify the cause of the low sodium. When the etiology is a correctly administered medication, the adverse effect T-code carries that detail. Omitting the T-code leaves the record without a documented cause. When the provider documents SIADH, E22.2 is assigned rather than E87.1 under the Excludes1 note at E87.1. When hyponatremia accompanies heart failure, the heart failure code carries clinical detail that E87.1 cannot.

Documentation checklist to support E87.1

Chart elements that support code selection and reduce specificity-driven denials.

Provider documentation of hyponatremia or hypo-osmolality as a confirmed clinical diagnosis.

A laboratory value supporting hyponatremia, a serum sodium result below the reference range of 135 mEq/L.

Clinical acknowledgment that the finding was evaluated, monitored or treated at the encounter.

Documentation of the etiology where known, such as a drug or volume overload, to support correct sequencing. SIADH directs the encounter to E22.2 instead.

The specific medication where hyponatremia is an adverse effect, to support the matching adverse effect T-code.

For inpatient encounters, the treatment rendered, such as intravenous sodium replacement or fluid restriction.

For outpatient encounters, documentation that the provider addressed the finding. A laboratory report alone is insufficient.

FAQ

Is E87.1 billable?

Yes. E87.1 is a single billable code with no child codes. Every presentation of hyponatremia and hypo-osmolality is captured under E87.1 when hyponatremia is the reported diagnosis.

What is the difference between E87.1 and E87.0?

The two codes run in opposite directions. E87.1 is hypo-osmolality and hyponatremia, meaning low serum sodium. E87.0 is hyperosmolality and hypernatremia, meaning elevated serum sodium.

Does E87.1 map to an HCC under CMS-HCC v28?

No. E87.1 does not map to any Hierarchical Condition Category in v28 or in the prior v24 model. Electrolyte and fluid balance disorders are not risk adjusting conditions. Reporting E87.1 adds no risk score weight.

Why does the code title say hypo-osmolality when the diagnosis is hyponatremia?

The official tabular description is hypo-osmolality and hyponatremia, and it covers both terms. The search term hyponatremia maps to E87.1. The full code title should be used in coding contexts.

How do I sequence drug-induced hyponatremia?

Sequence E87.1 first as the nature of the adverse effect. The adverse effect code from T36–T50 follows it (Guideline I.C.19.e.5.a). Loop diuretics such as furosemide take T50.1X5A. Thiazide diuretics take T50.2X5A. A T-code is not acceptable as the principal diagnosis in an adverse effect scenario.

Can I code E87.1 from a laboratory result alone?

No. The provider must acknowledge the finding as a diagnosis or a condition being addressed at the encounter. A laboratory report without clinical documentation that the finding was evaluated does not support the code.

Does E87.1 sequence differently inpatient and outpatient?

Outpatient coding takes E87.1 when the provider documents hyponatremia as a diagnosed and addressed condition. Inpatient coding follows UHDDS guidelines. Severe symptomatic hyponatremia may drive the principal diagnosis. Incidental hyponatremia discovered and treated is reported as an additional diagnosis.

Did E87.1 change in FY2027?

No. E87.1 is unchanged in the FY2027 update, effective 1 October 2026. The FY2027 update adds and deletes no codes in category E87. Its only tabular change in the category rewords the Code first note at E87.22, chronic metabolic acidosis. No codes were added to or deleted from category E87 in FY2026.

Related codes

Codes in the same family as E87.1. Only codes with a published page render as links.

Sibling codes in the E87 family

E87.0: Hyperosmolality and hypernatremia.
E87.2-: Acidosis.
E87.3: Alkalosis.
E87.4: Mixed disorder of acid-base balance.
E87.5: Hyperkalemia.
E87.6: Hypokalemia.
E87.7-: Fluid overload.
E87.8: Other disorders of electrolyte and fluid balance, not elsewhere classified.

Additional codes to consider

Codes outside the E87 family considered alongside E87.1.

Companion: code alongside when documented

T50.1X5A: Adverse effect of loop [high-ceiling] diuretics, initial encounter, sequenced after E87.1 under Guideline I.C.19.e.5.a.
T50.2X5A: Adverse effect of carbonic-anhydrase inhibitors, benzothiadiazides and other diuretics, initial encounter, sequenced after E87.1 under Guideline I.C.19.e.5.a.

Consider instead

E87.0: Hyperosmolality and hypernatremia, where serum sodium is elevated rather than low.
E87.6: Hypokalemia, where the documented disturbance is low potassium.
E87.5: Hyperkalemia, where the documented disturbance is elevated potassium.
E86.0: Dehydration.
E23.2: Diabetes insipidus, an Excludes1 condition that is not reported with E87.1 for the same condition.
E22.2: Syndrome of inappropriate secretion of antidiuretic hormone, an Excludes1 condition at E87.1.
O21.1: Hyperemesis gravidarum with metabolic disturbance.
O08.5: Metabolic disorders following an ectopic and molar pregnancy.

Sources

All claims on this page derive from government and professional-standards sources.

National Center for Health Statistics. (2026). International classification of diseases, tenth revision, clinical modification (ICD-10-CM) FY2027 files. https://www.cdc.gov/nchs/icd/icd-10-cm/files.html
Centers for Medicare & Medicaid Services and National Center for Health Statistics. (2026). ICD-10-CM official guidelines for coding and reporting FY2027. https://ftp.cdc.gov/pub/Health_Statistics/NCHS/Publications/ICD10CM/2027/ICD-10-CM-October-1-2026-FY27-Guidelines.pdf
Centers for Medicare & Medicaid Services. (2025). 2026 model software/ICD-10 mappings. https://www.cms.gov/medicare/payment/medicare-advantage-rates-statistics/risk-adjustment/2026-model-software-icd-10-mappings
Centers for Medicare & Medicaid Services. (2026). MS-DRG classifications and software, version 44. https://www.cms.gov/medicare/payment/prospective-payment-systems/acute-inpatient-pps/ms-drg-classifications-and-software