E83.42: Hypomagnesemia

The ICD-10-CM code for hypomagnesemia, low serum magnesium that the clinician has documented as a diagnosis rather than reported as a laboratory value alone.

Billable

FY2027

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At a glance

Code Family

E83.4: Disorders of magnesium metabolism.

Condition

Hypomagnesemia

Billable

Yes, valid for HIPAA-covered transactions with no further characters required.

HCC Mapping (v28)

None, carrying no risk adjustment weight under CMS-HCC v28 or CMS-HCC v24.

DRG Grouping

MS-DRG 640 or 641 in MDC 10 under the FY2027 MS-DRG v44 grouper, with the tier set by whether a secondary diagnosis carries an MCC.

Effective Date

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

Which E83.4 code do I assign?

The E83.4 family splits on the direction of the magnesium abnormality rather than on its cause. Assign E83.42 only where the clinician has documented low serum magnesium as a diagnosis.

When documentation says…

Code

Serum magnesium below the reference range, documented as hypomagnesemia

E83.42

Low serum magnesium recorded by the clinician as a diagnosis. The note must carry hypomagnesemia or low magnesium as a clinical conclusion, not a laboratory value standing on its own.

Elevated serum magnesium

E83.41

Hypermagnesemia. Assign it when the note documents an elevated rather than a low serum magnesium.

Magnesium disorder, direction not stated

E83.40

Disorders of magnesium metabolism, unspecified. Used where the note records a magnesium disorder without stating whether the level is high or low.

Specified magnesium disorder that is neither hypomagnesemia nor hypermagnesemia

E83.49

Other disorders of magnesium metabolism. Reserved for specified magnesium disorders not classified elsewhere.

Low magnesium attributed to inadequate dietary intake

E61.2

Magnesium deficiency, classified under nutritional deficiencies rather than metabolic disorders. Provider documentation and clinical context decide between E61.2 and E83.42.

Tabular notes

EXCLUDES1

Category E83 carries Excludes1 notes that E83.42 inherits. An Excludes1 note means the excluded condition is not reported with E83.42 for the same condition. Guideline I.A.12.a allows both codes when the conditions are unrelated. Dietary mineral deficiency, E58–E61, covers presentations classified as a nutritional deficiency. Parathyroid disorders, E20–E21, cover mineral metabolism disturbed by parathyroid disease. Vitamin D deficiency, the E55 codes, is classified separately. Where the clinical presentation fits one of these categories, assign the code from that category rather than E83.42. Category E61 carries the reciprocal Excludes1 note for disorders of mineral metabolism (the E83 codes).

FY2027: EFFECTIVE 1 OCTOBER 2026

The FY2027 codes and tabular addenda make no addition, deletion or revision in category E83. E83.42 is unchanged in the FY2027 update. No FY2026 changes were identified for E83.42 or for the parent E83.4 subcategory. The four codes under E83.4 and the Excludes1 notes at category E83 are unchanged.

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+)

E83.42 does not map to any HCC category under CMS-HCC v28.

E83.42 did not map to any HCC category under CMS-HCC v24 either. The v24 to v28 transition changed nothing for this code.

Hypomagnesemia contributes no risk adjustment weight to a Medicare Advantage risk score.

Where an HCC-bearing cause is documented, such as alcohol dependence, the risk adjustment value sits with that condition code rather than with E83.42.

MS-DRG v44 (FY2027)

E83.42 as a principal inpatient diagnosis groups to MS-DRG 640 or 641 in MDC 10 under the FY2027 MS-DRG v44 grouper.

MS-DRG 640 is miscellaneous disorders of nutrition, metabolism, fluids and electrolytes with MCC. MS-DRG 641 is the same group without MCC. The tier depends on whether a secondary diagnosis carries an MCC.

As a secondary diagnosis, E83.42 carries no CC or MCC designation in the FY2027 severity lists.

Medicare and commercial payers

E83.42 is a valid Medicare diagnosis code.

Intravenous magnesium replacement may require a documented and coded hypomagnesemia diagnosis to support medical necessity.

WHERE SPECIFICITY IS LOST

E83.40, disorders of magnesium metabolism unspecified, records a magnesium disorder without stating its direction. Where the note documents a low serum magnesium, E83.42 is the specific code.

Reporting the electrolyte finding alone, without the documented underlying cause, leaves any HCC-bearing etiology unreported for the encounter.

Assigning E83.42 where the documented etiology is primarily nutritional bypasses E61.2, magnesium deficiency.

Documentation checklist to support E83.42

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

Hypomagnesemia or low magnesium stated as a clinical diagnosis in the provider documentation.

A serum magnesium result below the laboratory reference range, supporting the documented diagnosis.

The underlying etiology where it is known, such as gastrointestinal losses, renal wasting, nutritional deficiency or chronic alcoholism.

The contributing medication where one is documented, such as a loop diuretic, a proton pump inhibitor or an aminoglycoside.

Evidence that the hypomagnesemia was evaluated, treated or monitored during the encounter, to support reporting it as a secondary inpatient diagnosis.

The manifestation under treatment where the admission is driven by hypomagnesemia, such as cardiac arrhythmia or neuromuscular irritability.

Confirmation that the presentation is not better classified as a nutritional magnesium deficiency, which directs the encounter to E61.2.

For an outpatient encounter, documentation that hypomagnesemia is the reason for the visit or a significant finding being addressed.

FAQ

Is E83.42 a billable code?

Yes. E83.42 is a valid billable code for HIPAA-covered transactions. No further characters are required.

When do I use E61.2 instead of E83.42?

E61.2, magnesium deficiency, sits under nutritional deficiencies and may apply where the etiology is primarily dietary. E83.42, hypomagnesemia, sits under metabolic disorders and applies where the clinician documents hypomagnesemia as the diagnosis. Provider documentation and clinical context decide between the two. The Excludes1 note at category E83 references the E58–E61 dietary mineral deficiency range.

Does E83.42 map to an HCC?

No. E83.42 carries no HCC category under CMS-HCC v28, and it carried none under CMS-HCC v24. It contributes no risk adjustment weight. Where an underlying condition is documented, the risk adjustment value sits with that condition code.

Can I assign E83.42 from a laboratory result alone?

No. The provider must document hypomagnesemia as a clinical diagnosis or clinical conclusion. A low magnesium value supports the finding. The documented diagnosis governs code assignment.

Should I code the underlying cause alongside E83.42?

Yes, where it is established and documented. No tabular note at E83.42 restricts reporting it with its etiology. The Excludes1 conditions at category E83 are the exception. Sequence the condition that is the focus of treatment first, per the ICD-10-CM sequencing guidelines.

How is E83.42 sequenced for an inpatient admission?

Where the patient is admitted for treatment of hypomagnesemia and its manifestations, E83.42 may be appropriate as the principal diagnosis under UHDDS rules. That is the condition established after study to be chiefly responsible for the admission. Where hypomagnesemia is instead a complication or a comorbidity of the stay, report it as a secondary diagnosis when it is evaluated, treated or monitored.

Does E83.42 carry a Use Additional or Code First instruction?

No. E83.42 carries no tabular level Code First or Use Additional instruction. Where the underlying etiology is known and documented, report both E83.42 and that condition, sequencing per the ICD-10-CM guidelines.

What changed for E83.42 in FY2027?

E83.42 is unchanged in the FY2027 update, effective 1 October 2026. The FY2027 codes and tabular addenda make no addition, deletion or revision in category E83. No FY2026 changes were identified for E83.42 or for the parent E83.4 subcategory. The code structure and the Excludes1 notes at category E83 are unchanged.

Related codes

Codes in the same family as E83.42. Only codes with a published page render as links.

Sibling codes in the E83.4 family

E83.40: Disorders of magnesium metabolism, unspecified.
E83.41: Hypermagnesemia.
E83.49: Other disorders of magnesium metabolism.

Additional codes to consider

Codes outside the E83.4 family considered alongside E83.42.

Consider instead

E61.2: Magnesium deficiency, where the etiology is primarily nutritional.
E58: Dietary calcium deficiency, within the dietary mineral deficiency range excluded at E83.
E20-: Hypoparathyroidism, a category that requires further characters, where a parathyroid disorder accounts for the mineral disturbance.
E21-: Hyperparathyroidism and other disorders of parathyroid gland, a category that requires further characters.
E55-: Vitamin D deficiency, a category that requires further characters, classified separately from disorders of mineral metabolism.

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