E87.1

🧬 ICD-10-CM E87.1 — Hypo-osmolality and Hyponatremia

Billable Code Confirmed

ICD-10-CM E87.1 is a valid, billable 4-character ICD-10-CM code for FY2025. The code structure is: E87 (category — Other disorders of fluid, electrolyte and acid-base balance) + .1 (4th character — Hypo-osmolality and hyponatremia). No 5th, 6th, or 7th character is required or available — E87.1 is a terminal (leaf-level) code at 4 characters. This code is valid for claims submission from October 1, 2024 through September 30, 2025.

Non-Billable Parent Code — Never Submit This

• ❌ E87 — 3-character category header — missing the specific condition character; not billable on its own

Always submit E87.1 (all 4 characters) when the provider has documented hyponatremia and/or hypo-osmolality as a confirmed diagnosis. Note: unlike many ICD-10-CM codes in Chapter 4, E87.1 requires only 4 characters — submitting a 5th character constitutes an invalid code.

Critical Distinction: E87.1 vs. E22.2 (SIADH) — Excludes1 at the E87.1 Code Level

The single most important coding rule for E87.1 is its Excludes1 relationship with E22.2 — Syndrome of Inappropriate Secretion of Antidiuretic Hormone (SIADH). The ICD-10-CM Tabular List places an Excludes1 note directly at the E87.1 code level:

“Excludes1: syndrome of inappropriate secretion of antidiuretic hormone (E22.2)”

This means E87.1 and E22.2 can NEVER appear together on the same claim. When the provider documents SIADH as the cause of the low sodium, E22.2 is the correct and only code — do NOT code E87.1 in addition. E22.2 captures the complete clinical picture when SIADH is diagnosed. However:

• If the provider documents “hyponatremia, etiology under investigation” or “hyponatremia, likely SIADH but not confirmed” → code E87.1 only (SIADH not confirmed)
• If the provider documents “SIADH with hyponatremia” → code E22.2 only (Excludes1 prohibits E87.1)
• If the provider documents “hyponatremia, cause unknown” → code E87.1 only

Additional type distinction: E87.1 applies to true hypoosmolar hyponatremia (serum osmolality < 280 mOsm/kg). Do NOT use E87.1 for:

• Pseudohyponatremia (isotonic hyponatremia from hyperlipidemia or hyperproteinemia — osmolality is normal; this is a lab artifact)
• Hypertonic hyponatremia (from hyperglycemia — glucose-diluted sodium; code the hyperglycemia/DM; E87.1 does not apply when osmolality is elevated)

🔍 Code Description

ICD-10-CM E87.1 classifies hypo-osmolality and hyponatremia — an abnormally low serum sodium concentration, generally defined as serum sodium (Na⁺) below 135 mEq/L, accompanied by reduced plasma osmolality (< 280 mOsm/kg). Sodium is the dominant extracellular cation and primary determinant of extracellular fluid osmolality; its depletion or dilution disrupts osmotic equilibrium, driving water into cells and causing cellular swelling.

Hyponatremia is one of the most common electrolyte disorders in hospitalized patients — affecting approximately 15-30% of inpatients — and one of the top causes of DRG 640 admissions nationally. Clinical severity scales with the rate of fall and absolute level:

• Mild (130-134 mEq/L): Often asymptomatic; mild nausea, fatigue
• Moderate (125-129 mEq/L): Headache, confusion, altered gait, lethargy
• Severe (< 125 mEq/L): Cerebral edema, seizures, coma, respiratory arrest — medical emergency
• Chronic (any level, > 48 hours): Neurons adapt; rapid correction more dangerous than condition

The three volume-status subtypes drive treatment selection and should always be documented and coded with the underlying condition:

Volume Status	Mechanism	Underlying Causes (Code Separately)	Treatment
Hypovolemic	Na + water lost; more Na lost than water	Diuretics (T50.1X5A), vomiting, diarrhea, bleeding, Addison’s disease	Isotonic NS resuscitation
Euvolemic	Water retained; Na normal or slightly depleted	SIADH (E22.2 — Excludes1!), hypothyroidism (E03.9), adrenal insufficiency (E27.49), psychogenic polydipsia	Fluid restriction; vaptans
Hypervolemic	Both Na + water retained; proportionally more water	CHF (I50.x), cirrhosis (K74.6x), nephrotic syndrome (N04.x), CKD (N18.x)	Fluid restriction; loop diuretics; vaptans

Note

Osmotic Demyelination Syndrome (ODS) — A Feared Complication of Overcorrection. If serum sodium is corrected faster than 10-12 mEq/L per 24 hours (or 8 mEq/L per 24 hours in high-risk patients), osmotic demyelination syndrome (formerly called central pontine myelinolysis) can occur — a catastrophic, often irreversible neurological complication. If the provider documents ODS as a complication of hyponatremia treatment, code G37.2 (Central pontine myelinolysis, or other demyelinating disease of CNS) as an additional diagnosis alongside E87.1. This is a critical secondary diagnosis that should trigger a CDI query if neurological deterioration during treatment is documented.

🌳 Code Tree / Hierarchy

E87  Other disorders of fluid, electrolyte and acid-base balance ❌ Non-billable
     [Excludes1 at E87 category level: diabetes insipidus (E23.2);
      electrolyte imbalance with hyperemesis gravidarum (O21.1);
      electrolyte imbalance following ectopic/molar pregnancy (O08.5);
      familial periodic paralysis (G72.3)]
│
├── E87.0  Hyperosmolality and hypernatremia ✅ Billable
│          (High sodium — opposite of E87.1; not coded together)
│
├── E87.1  HYPO-OSMOLALITY AND HYPONATREMIA ◀ THIS CODE ✅ Billable
│          [Excludes1 directly at E87.1: SIADH (E22.2) — NEVER code with E87.1]
│
├── E87.2  Acidosis ✅ Billable
│          (Includes: metabolic acidosis, respiratory acidosis, lactic acidosis)
│
├── E87.3  Alkalosis ✅ Billable
│
├── E87.4  Mixed disorder of acid-base balance ✅ Billable
│
├── E87.5  Hyperkalemia ✅ Billable
│
├── E87.6  Hypokalemia ✅ Billable (often co-occurs with E87.1 in diuretic use)
│
├── E87.7  Fluid overload ❌ Non-billable header
│   ├── E87.70  Fluid overload, unspecified ✅
│   ├── E87.71  Transfusion-associated circulatory overload (TACO) ✅
│   └── E87.79  Other fluid overload ✅
│
└── E87.8  Other disorders of electrolyte and fluid balance, NEC ✅ Billable

Frequently co-coded related codes:

E22.2   SIADH — Excludes1 to E87.1; code INSTEAD when SIADH is confirmed diagnosis
E86.0   Dehydration — common with hypovolemic hyponatremia; code together when both documented
E86.1   Hypovolemia — volume depletion with Na loss; separately reportable
E87.6   Hypokalemia — may co-occur (thiazide diuretics cause both); code both when documented
E83.42  Hypomagnesemia — co-occurring electrolyte disorder; code separately when documented
E83.51  Hypocalcemia — co-occurring; code separately when documented
G37.2   Central pontine myelinolysis (ODS) — complication of overcorrection; code if documented

✅ Includes

The following clinical terms, documentation phrases, and clinical scenarios map to E87.1:

• Hyponatremia (serum Na < 135 mEq/L) — when physician-documented as a diagnosis
• Hypo-osmolality (serum osmolality < 280 mOsm/kg) — when physician-documented
• Hypovolemic hyponatremia (diuretic-induced, GI losses, adrenal insufficiency — code cause separately)
• Hypervolemic hyponatremia (heart failure, cirrhosis, nephrotic syndrome, CKD — code underlying condition separately)
• Euvolemic hyponatremia when SIADH is NOT confirmed (psychogenic polydipsia, hypothyroidism, adrenal insufficiency — code cause additionally; use E22.2 when SIADH IS confirmed)
• Beer potomania (extreme low-solute beer intake causing dilutional hyponatremia — code alcohol use additionally)
• Exercise-associated hyponatremia (excessive free water intake)
• Drug-induced hyponatremia (thiazides, SSRIs, NSAIDs, antiepileptics — add T-series adverse effect code)
• Post-operative hyponatremia when documented by provider

❌ Excludes

Excludes1 Directly at E87.1 — Cannot EVER be coded WITH E87.1

• E22.2 — Syndrome of inappropriate secretion of antidiuretic hormone (SIADH): When SIADH is the documented, confirmed diagnosis, use E22.2 exclusively — this is a dedicated code for the specific ADH hypersecretion disorder that causes euvolemic hyponatremia; E87.1 and E22.2 are mutually exclusive per Excludes1

Excludes1 at E87 Category Level — Cannot be coded WITH any E87.x code

• Diabetes insipidus (E23.2) — a distinct ADH deficiency disorder causing hypernatremia, not hyponatremia; categorically incompatible
• Electrolyte imbalance associated with hyperemesis gravidarum (O21.1) — use obstetric code instead; E87.x cannot be used concurrently
• Electrolyte imbalance following ectopic or molar pregnancy (O08.5) — same principle; obstetric code takes precedence
• Familial periodic paralysis (G72.3) — neuromuscular condition; Excludes1 at E87 level
• Metabolic acidemia in newborn (P19.9) — neonatal code takes precedence; do not use E87.x for neonatal conditions

Excludes2 — Can be coded together with E87.1 if both documented

• Disorders of mineral metabolism (E83.xx) — including hypomagnesemia (E83.42) and hypocalcemia (E83.51); code all documented electrolyte disorders separately
• Dehydration (E86.0) / Hypovolemia (E86.1) — may coexist and should be separately coded in hypovolemic hyponatremia
• Malnutrition (E40-E46) — commonly underlying; separately coded and HCC-bearing
• Heart failure (I50.x), cirrhosis (K74.x), CKD (N18.x) — underlying causes; all separately coded and frequently HCC-bearing

🛠️ CPT Procedural Crosswalk — wRVU & Assistant Payable Status

The following CPT and HCPCS codes are most commonly associated with the evaluation and treatment of E87.1. Management ranges from lab monitoring and fluid restriction in mild cases to intensive IV therapy and critical care in severe symptomatic hyponatremia.

CPT / HCPCS	Description	wRVU (Facility)	Asst. Surgeon Payable?	Co-Surgeon Payable?
96365	Intravenous infusion, therapeutic/prophylactic/diagnostic; initial, up to 1 hour — for hypertonic saline (3% NaCl), isotonic NS, or IV vaptan (conivaptan) infusion	0.17	❌ No	❌ No
96366	IV infusion, each additional hour (add-on to 96365) — 3% NaCl correction infusions may run 8-24+ hours	0.10	❌ No	❌ No
J7030	Infusion, normal saline solution, 250 ml — for hypovolemic hyponatremia volume repletion with isotonic saline	N/A (supply)	N/A	N/A
J7040	Infusion, normal saline solution, 500 ml — isotonic NS; report units based on total volume administered	N/A (supply)	N/A	N/A
J0218	Injection, conivaptan hydrochloride, 1 mg — IV vasopressin receptor antagonist (vaptan) approved for euvolemic and hypervolemic hyponatremia in hospitalized patients; report units per mg administered	N/A (drug)	N/A	N/A
84295	Sodium — serum; primary diagnostic and monitoring lab for hyponatremia; report with each ordered level	0.00 (lab)	N/A	N/A
84210	Osmolality — blood (serum); confirms true hypoosmolality; differentiates true from pseudo-hyponatremia; essential for E87.1 diagnostic workup	0.00 (lab)	N/A	N/A
83935	Osmolality — urine; critical for SIADH vs. other euvolemic hyponatremia differentiation; urine osmolality > 100 mOsm/kg suggests SIADH or renal Na loss	0.00 (lab)	N/A	N/A
84300	Sodium — urine; used to calculate FENa (Fractional Excretion of Sodium) and urine Na concentration; differentiates renal vs. extrarenal Na loss in hypovolemic hyponatremia	0.00 (lab)	N/A	N/A
80048	Basic Metabolic Panel — includes serum Na, K, Cl, CO2, creatinine, glucose, BUN, Ca; standard monitoring panel	0.00 (lab)	N/A	N/A
80051	Electrolyte panel — Na, K, Cl, CO2; focused electrolyte monitoring without full BMP	0.00 (lab)	N/A	N/A
93000	Electrocardiogram, routine; with interpretation and report — for monitoring; severe hyponatremia with cardiac symptoms or co-existing hypokalemia (E87.6) elevates arrhythmia risk	0.17	❌ No	❌ No
99291	Critical care, first 30-74 minutes — for severe symptomatic hyponatremia (Na < 120 with seizures, coma, respiratory compromise)	4.50	❌ No	❌ No
99292	Critical care, each additional 30 minutes (add-on to 99291)	2.25	❌ No	❌ No

⚠️ Hypertonic Saline (3% NaCl) Billing Note: There is no universally assigned, publicly available HCPCS J-code specific to 3% NaCl hypertonic saline. In the inpatient (Part A/DRG) setting, 3% NaCl is bundled into the DRG — no separate billing. In the outpatient/ED (Part B) setting, hypertonic saline is typically billed by the facility as an IV infusion administration service (96365/96366) with the 3% NaCl solution as a separately chargeable supply under facility billing practices. Verify billing with your facility coding compliance team and MAC policy before submitting.

⚠️ Conivaptan (J0218) Important Note: Conivaptan (Vaprisol®) is approved by FDA specifically for inpatient use only in hospitalized patients with euvolemic or hypervolemic hyponatremia. It is an IV CYP3A4 inhibitor requiring close monitoring. The paired diagnosis codes are E87.1 (when SIADH is NOT confirmed as etiology) for euvolemic cases, or the hypervolemic underlying etiology code (I50.x, K74.x, N18.x) for hypervolemic cases. Tolvaptan (oral) has no specific J-code for standard outpatient use and is typically billed via pharmacy/drug benefit.

💊 Coding Scenarios

Scenario 1 — Hyponatremia in Decompensated Heart Failure (Hypervolemic), Inpatient

Clinical Vignette: A 78-year-old male with systolic heart failure (LVEF 30%) is admitted with worsening dyspnea, 15 lb weight gain, and altered mental status. Serum Na is 121 mEq/L, BNP is 2,400 pg/mL. The cardiologist documents “severe hypervolemic hyponatremia secondary to decompensated heart failure.” IV loop diuretics, fluid restriction, and conivaptan infusion are initiated.

CPT / HCPCS Codes:

• 96365 — IV infusion, initial hour (conivaptan)
• 96366 × 23 — Continuous 24-hour conivaptan infusion (remaining hours)
• J0218 × 20 — Conivaptan hydrochloride, 20 mg total (per dosing protocol; 20 units of 1 mg/unit)
• 84295 × 4 — Serial serum sodium monitoring (Q6h)
• 80048 — BMP (BUN, creatinine monitoring)
• 93000 — EKG

ICD-10-CM:

• Principal Dx: I50.20 — Unspecified systolic (congestive) heart failure (the condition driving the admission; the hypervolemia and hyponatremia are manifestations)
• Secondary Dx: E87.1 — Hypo-osmolality and hyponatremia (confirmed hypervolemic hyponatremia; separately managed with fluid restriction and conivaptan)

🏥 Inpatient Coder Tip: Sequence the underlying condition driving the admission (decompensated CHF) as principal. E87.1 is secondary here — a complication of the heart failure. I50.20 has a CC/MCC crosswalk impact depending on grouper version; document the specific CHF type (systolic vs. diastolic, acute vs. chronic vs. acute-on-chronic) to maximize DRG accuracy. Never miss coding E87.1 as a secondary — it reflects significant clinical complexity and comorbidity burden even when not principal.

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Scenario 2 — Severe Symptomatic Hyponatremia Requiring 3% Saline, ICU

Clinical Vignette: A 44-year-old female marathon runner is brought to the ED by EMS after collapsing at mile 22 with a tonic-clonic seizure. Serum Na is 118 mEq/L, serum osmolality 238 mOsm/kg. Glucose and thyroid function are normal. The emergency physician documents “severe symptomatic hyponatremia, likely exercise-associated; acute seizure.” She is admitted to the ICU for 3% NaCl correction, seizure monitoring, and close sodium reassessment q2h.

CPT / HCPCS Codes:

• 99291 — Critical care, first 30-74 minutes (severe symptomatic hyponatremia with active seizure)
• 99292 × 2 — Additional critical care time
• 96365 — IV infusion initial hour (3% NaCl administered via infusion)
• 96366 × 5 — Continued hypertonic saline infusion (additional hours)
• 84295 × 8 — Serum sodium q2h monitoring
• 84210 — Serum osmolality (confirms true hypoosmolality; excludes pseudohyponatremia)
• 93000 — EKG (baseline arrhythmia screening)

ICD-10-CM:

• Principal Dx: E87.1 — Hypo-osmolality and hyponatremia (the reason for admission after study; confirmed on osmolality)
• Secondary Dx: R56.9 — Unspecified convulsions (active seizure at presentation; manifestation of severe hyponatremia)
• Secondary Dx: Z16.10 — or consider underlying cause documentation (exercise-associated; provider should document this explicitly for clarity)

🏥 Inpatient Coder Tip: When seizures are attributed to hyponatremia in the documentation, code both E87.1 and R56.9 — the seizure is a manifestation but should be separately coded per the UHDDS rule that any condition monitored or evaluated is reportable. R56.9 does NOT carry MCC status here, but documenting status epilepticus (G41.9) — if present — DOES carry MCC status and significantly elevates DRG. Query the provider if seizure activity continued or required antiepileptic treatment.

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Scenario 3 — SIADH vs. E87.1: CDI Sequencing Decision

Clinical Vignette: A 65-year-old male with small cell lung cancer is admitted with Na 126 mEq/L. Urine osmolality is 480 mOsm/kg (elevated), urine sodium is 65 mEq/L. Serum osmolality is 265 mOsm/kg. The patient is euvolemic. The hospitalist’s admission note documents “hyponatremia, rule out SIADH given lung cancer.” On day 2, the note states “SIADH confirmed — ADH elevation consistent with paraneoplastic syndrome.”

ICD-10-CM on Day 1 (admission):

• E87.1 — Hypo-osmolality and hyponatremia (“rule out SIADH” = not confirmed at admission; code the sign/symptom E87.1)

ICD-10-CM at Discharge (after SIADH confirmed):

• Principal Dx: E22.2 — Syndrome of inappropriate secretion of antidiuretic hormone (ONLY — E87.1 CANNOT be coded with E22.2 per Excludes1; once SIADH is confirmed, E22.2 fully captures the clinical picture)
• Secondary Dx: C34.10 — Malignant neoplasm of upper lobe, bronchus or lung, unspecified side (small cell lung cancer, the underlying paraneoplastic cause)

🏥 CDI/Inpatient Coder Tip: This scenario is one of the most common CDI opportunities with E87.1. On admission, “rule out SIADH” is NOT a confirmed diagnosis — code E87.1 per inpatient coding guidelines (possible conditions on admission can be coded as confirmed at inpatient level, BUT only when they are still “possible” at discharge per UHDDS). Once the attending documents “SIADH confirmed” at discharge, switch to E22.2 exclusively and remove E87.1. The Excludes1 is absolute — do NOT code both. Also capture the oncological etiology; C34.x may carry significant comorbidity impact on DRG weighting.

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Scenario 4 — Drug-Induced Hyponatremia (SSRI/Thiazide), Outpatient

Clinical Vignette: A 72-year-old female on hydrochlorothiazide and sertraline is seen in the nephrology office for fatigue, nausea, and confusion. Labs show Na 129 mEq/L. Urine osmolality is 380 mOsm/kg. She is euvolemic. The nephrologist documents “hyponatremia, adverse effect of both hydrochlorothiazide and sertraline; both medications are being held.”

CPT / HCPCS Codes:

• 99214 — Office visit, moderate medical decision making
• 84295 — Serum sodium
• 84210 — Serum osmolality
• 83935 — Urine osmolality (confirms euvolemic SIADH-like picture)

ICD-10-CM:

• E87.1 — Hypo-osmolality and hyponatremia (the confirmed diagnosis; note: provider has NOT diagnosed SIADH, so E22.2 does NOT apply here)
• T50.2X5A — Adverse effect of carbonic-anhydrase inhibitors, benzothiadiazides and other diuretics (for hydrochlorothiazide adverse effect), initial encounter
• T43.225A — Adverse effect of selective serotonin reuptake inhibitors, initial encounter (for sertraline adverse effect)
• Z79.899 — Other long term (current) drug therapy (for documentation of chronic SSRI and diuretic use)

🏥 Outpatient Coder Tip: Code E87.1 as the primary diagnosis code linked to the encounter. Add both adverse effect T-codes (T50.2X5A and T43.225A) since the provider has documented hyponatremia as a consequence of CORRECTLY administered medications — this is the “adverse effect” scenario (5th character = 5), NOT poisoning (1-4) or underdosing (6). Always capture adverse effect T-codes when the provider explicitly documents drug causation — this is an audit compliance and medical necessity requirement. Note that SIADH has NOT been confirmed here — the provider’s documented diagnosis is “hyponatremia, adverse effect of…” which warrants E87.1, not E22.2.

⚠️ Coding Pitfalls and Tips

	Pitfall or Tip
❌	E22.2 (SIADH) and E87.1 are NEVER coded together — ever. Excludes1 is located directly at the E87.1 code level in the Tabular List. When SIADH is confirmed by the provider, use E22.2 exclusively and remove E87.1 from the claim. This is the single most audited coding error with hyponatremia
❌	Do not code E87.1 from a serum sodium lab value alone. A Na of 132 mEq/L in the results does not authorize E87.1. The attending, hospitalist, or treating provider must document “hyponatremia,” “low sodium,” “hypo-osmolality,” or equivalent clinical language in their assessment or plan
❌	Do not use E87.1 for pseudohyponatremia or hypertonic hyponatremia. When low Na is present with normal or elevated serum osmolality (e.g., hyperglycemia, severe hyperlipidemia), E87.1 does not apply — the code specifically pairs hypo-osmolality with the low sodium. Code the underlying condition instead
❌	Do not use E87.1 for neonatal electrolyte disorders. The E87 category Excludes1 note excludes metabolic acidemia in newborns; use neonatal-specific P-codes (P74.x series) for neonatal electrolyte disturbances
❌	Do not miss the DRG 640 MCC opportunity. E87.1 as principal maps to DRG 641 without MCC — but hyponatremia admissions frequently involve MCC-bearing secondary conditions (sepsis, hepatic failure, respiratory failure). DRG 640 vs. 641 can represent thousands of dollars in reimbursement difference; always query for MCCs when documented in the body of the chart but not as a coded secondary diagnosis
✅	Always code the underlying etiology separately when documented. Heart failure (I50.x — HCC 85/86), cirrhosis (K74.6x — HCC), CKD (N18.x — HCC 136/137), adrenal insufficiency (E27.49), hypothyroidism (E03.9) — these are the HCC-bearing conditions that give E87.1 its clinical context and risk adjustment value
✅	Code all co-existing electrolyte disorders separately. E87.1 + E87.6 (hypokalemia from diuretics) + E83.42 (hypomagnesemia) should each appear when documented and managed — do NOT collapse to a single unspecified code
✅	Watch for ODS/CPM as a complication. If the patient develops neurological deterioration during hyponatremia correction (confusion, dysarthria, dysphagia), query the provider about osmotic demyelination syndrome (G37.2). This is a potentially catastrophic complication from overcorrection and is separately codeable — and a significant clinical quality indicator
✅	Drug-induced hyponatremia: always add the adverse effect T-code. SSRIs, thiazide diuretics, carbamazepine, oxcarbazepine, NSAIDs, vincristine, cyclophosphamide — all are documented causes of drug-induced hyponatremia. Always capture the T-series adverse effect code alongside E87.1 when the provider documents drug causation
✅	In the inpatient setting, “possible” or “probable” hyponatremia CAN be coded. Per ICD-10-CM Official Guideline Section II.H, inpatient diagnoses documented as “possible,” “probable,” “suspected,” or “still to be ruled out” at the time of discharge may be coded as confirmed. This is unique to inpatient — in outpatient settings, code the sign/symptom only if SIADH or hyponatremia is not confirmed

📚 Sources

1. CMS/NCHS. ICD-10-CM Official Guidelines for Coding and Reporting, FY2025. Tabular List — E87.1; Section I.C.4 — Endocrine, Nutritional, and Metabolic Diseases; Section II.H — Uncertain Diagnoses (Inpatient); Section III — Reporting Additional Diagnoses.

2. World Health Organization / CMS. ICD-10-CM Tabular List of Diseases and Injuries, FY2025 Release. Category E87 — Other disorders of fluid, electrolyte and acid-base balance; E87.1 Excludes1 note: Syndrome of inappropriate secretion of antidiuretic hormone (E22.2).

3. Centers for Medicare & Medicaid Services. MS-DRG v37.2 / v41 (FY2025) Definitions Manual. MDC 10 — E871 (Hypo-osmolality and hyponatremia) confirmed in DRG 640/641 grouper table.

4. HIA (Health Information Associates). Most Common DRGs with CDI Recommendations 2021 — DRG 640 Analysis. Hyponatremia identified as top principal diagnosis driving DRG 640 admissions with MCC upgrade opportunities.

5. AAPC. ICD-10-CM Code E87.1 — Hypo-osmolality and Hyponatremia. Codify reference, Excludes1 notation at E87 and E87.1. FY2025.

6. American Medical Association (AMA). CPT 2024/2025 Professional Edition. Infusion codes 96365-96366; laboratory codes 84295, 84210, 83935, 84300, 80048, 80051.

7. RCMQuest / RevenueES / MDClarity. E87.1 ICD-10 Code — Hyponatremia Billing and Coding Guides. 2025-2026.

8. UCSF Hospital Handbook. Hyponatremia — Diagnosis, Classification, and Treatment Protocol. Clinical context for volume status classification, ODS risk, and correction rate guidelines.

9. PMC / NCBI. Validation of ICD-10 Codes Used to Identify the Main Reason for Hospitalization for Hyponatremia. Published February 2025. Confirms E87.1 as primary inpatient hyponatremia code with high positive predictive value.