Crystal's MCW Coder Hub🧠 ICD-10-CM R29.0 — Tetany
Billable Code Confirmed
ICD-10-CM R29.0 is a valid, billable 4-character ICD-10-CM code for FY2026. The code is fully specified:
R29(other symptoms and signs involving the nervous and musculoskeletal systems) +.0(tetany). All four characters are required for valid reporting.
Non-Billable Parent Code — Never Submit Alone
- ❌
R29— 3-character category header — non-billable; never submit aloneAlways submit R29.0 (all 4 characters) when tetany is the documented finding and no definitive etiology code fully captures the presentation.
Symptom Code — Four Hard Excludes 1 Entries; Know Them All
R29.0 has four Excludes 1 entries — the broadest Excludes 1 profile of any symptom code in the R25-R29 block. Before assigning R29.0, the coder must confirm the presentation is not:
- Hysterical tetany → F44.5 (conversion disorder — psychiatric etiology)
- Neonatal tetany → P71.3 (neonatal without Ca/Mg deficiency)
- Parathyroid tetany → E20.9 (or specific E20.x — hypoparathyroidism confirmed)
- Post-thyroidectomy tetany → E89.2 (post-surgical hypoparathyroidism)
If any of these four scenarios is documented and confirmed, assign that specific code — do not assign R29.0. These are hard Excludes 1 rules, not judgment calls.
Code Classification
ICD-10-CM Diagnosis Code — Fields for wRVU, assistant payable, and global period are not applicable. This is a symptom/sign code used across inpatient, outpatient, and ED settings when neuromuscular hyperexcitability consistent with tetany is documented and the specific etiology has not yet been established or does not map to a more defined code.
🔍 Code Description
ICD-10-CM R29.0 classifies tetany — a syndrome of involuntary neuromuscular hyperexcitability characterized by muscle cramps, spasms, and twitching caused by abnormally increased peripheral nerve irritability. It is a symptom-level code, appropriate when tetany is observed and documented by the clinician but the underlying etiology has not yet been established, or when the etiology does not fall into one of the four Excludes 1 categories.
The clinical hallmark of tetany is carpopedal spasm — involuntary flexion and adduction of the wrist and fingers (main d’accoucheur posture) with foot and toe extension — which is specifically listed as an included term under R29.0. Additional classic findings include Chvostek’s sign (facial muscle twitching elicited by tapping the facial nerve anterior to the ear) and Trousseau’s sign (carpopedal spasm induced by inflating a blood pressure cuff above systolic pressure for 3 minutes). Laryngospasm, bronchospasm, and generalized seizure-like activity can occur in severe cases.
R29.0 is most commonly encountered in the inpatient and ED settings as the initial code before electrolyte and endocrine workup is complete. In many inpatient encounters, the code is appropriate only transiently — once hypocalcemia (E83.51), hypomagnesemia (E83.42), alkalosis (E87.3), or another confirmed etiology is established, the etiology code typically replaces or supplements R29.0 as the principal diagnosis per ICD-10-CM Official Coding Guidelines.
🌳 Code Tree / Hierarchy
R29 — Other Symptoms and Signs Involving the Nervous and Musculoskeletal Systems ❌ Non-billable
│
├── R29.0 — Tetany ◀ THIS CODE ✅ Billable
├── R29.1 — Meningismus ✅ Billable
├── R29.2 — Abnormal Reflex ✅ Billable
├── R29.3 — Abnormal Posture ✅ Billable
├── R29.4 — Clicking Hip ✅ Billable
├── R29.5 — Transient Paralysis ✅ Billable
├── R29.6 — Repeated Falls ✅ Billable
├── R29.7- — NIHSS Score ✅ Billable (R29.700-R29.744)
├── R29.81- — Other Symptoms and Signs Involving the Nervous System ✅ Billable
└── R29.9- — Unspecified Symptoms and Signs Involving the Nervous and Musculoskeletal Systems ⚠️ Avoid — query specificity
R29.0 vs. R25.2 — Tetany vs. Cramp and Spasm
R29.0 is specifically for tetany — a syndrome of generalized neuromuscular hyperexcitability with a recognized clinical pattern (Chvostek’s, Trousseau’s, carpopedal spasm). It is not interchangeable with R25.2 (cramp and spasm), which captures isolated, localized muscle cramping without the systemic neuromuscular hyperexcitability picture. If the physician documents “tetany” or “carpopedal spasm” → R29.0. If the physician documents “leg cramp” or “muscle cramp” → R25.2 or M62.83x per documentation specificity.
✅ Includes
The following clinical documentation patterns map to R29.0 when the four Excludes 1 categories have been excluded:
- Tetany NOS — documented by the treating physician
- Carpopedal spasm — ICD-10-CM listed included term; characteristic hand/foot posturing of neuromuscular hyperexcitability
- Tetanic spasm — in non-neonatal, non-hysterical, non-parathyroid context
- Neuromuscular hyperexcitability documented as “tetany” — regardless of suspected metabolic cause, when no definitive etiology has been confirmed at time of coding
- Latent tetany — subclinical; positive Chvostek’s or Trousseau’s sign documented without overt carpopedal spasm
❌ Excludes
Excludes 1 — Cannot Be Coded Simultaneously with R29.0
| Code | Description | Note |
|---|---|---|
| F44.5 | Conversion disorder with seizures or convulsions (hysterical tetany) | Mutually exclusive — psychiatric/functional etiology; if hysterical tetany is confirmed, assign F44.5, not R29.0 |
| P71.3 | Neonatal tetany without calcium or magnesium deficiency | Mutually exclusive — neonatal presentation has its own perinatal code; R29.0 is for non-neonatal tetany |
| E20.9 | Hypoparathyroidism, unspecified (parathyroid tetany) | Mutually exclusive — when parathyroid gland failure is confirmed as the cause of tetany, assign E20.x; R29.0 is excluded |
| E89.2 | Post-procedural hypoparathyroidism (post-thyroidectomy tetany) | Mutually exclusive — post-surgical parathyroid failure has its own definitive code; R29.0 is excluded when E89.2 applies |
Excludes 1 — Four Hard Rules; E89.2 Is the Most Commonly Missed
The E89.2 Excludes 1 is the highest-risk pitfall in the inpatient and surgical settings. A patient who develops tetany after thyroidectomy or parathyroidectomy does not get R29.0 — the correct code is E89.2 (post-procedural hypoparathyroidism), which fully captures the post-surgical calcium-PTH axis dysfunction. Assigning R29.0 on a post-thyroidectomy encounter when tetany is present is an Excludes 1 violation. Check the surgical history before assigning R29.0.
📋 Clinical Overview
R29.0 vs. the Excludes 1 Codes — The Critical Decision Tree
The most important coding decision with R29.0 is determining whether one of the four Excludes 1 conditions is present. This requires a systematic review of the encounter type and physician documentation.
| Clinical Scenario | Correct Code | R29.0 Appropriate? |
|---|---|---|
| Tetany, etiology unknown or workup pending | R29.0 | ✅ Yes |
| Tetany with confirmed hypocalcemia — no parathyroid cause | R29.0 + E83.51 | ✅ Yes — hypocalcemia is NOT Excludes 1 |
| Tetany with confirmed hypomagnesemia | R29.0 + E83.42 | ✅ Yes — hypomagnesemia is NOT Excludes 1 |
| Tetany with confirmed metabolic alkalosis | R29.0 + E87.3 | ✅ Yes — alkalosis is NOT Excludes 1 |
| Tetany in a patient post-thyroidectomy | E89.2 | ❌ No — Excludes 1 |
| Tetany due to confirmed hypoparathyroidism | E20.x | ❌ No — Excludes 1 |
| Tetany in a neonate without Ca/Mg deficiency | P71.3 | ❌ No — Excludes 1 |
| ”Hysterical tetany” / conversion disorder | F44.5 | ❌ No — Excludes 1 |
Hypocalcemia Is NOT an Excludes 1 Entry
This is the single most important nuance for R29.0. E83.51 (hypocalcemia) is not in the Excludes 1 list — it is an etiology of tetany and may be coded alongside R29.0 when both are documented. The Excludes 1 entries are specifically the four condition-specific definitive codes that fully replace the symptom. Hypocalcemia with tetany → code both E83.51 and R29.0, sequencing per the reason for the encounter.
Pathophysiology
Tetany results from increased neuronal membrane excitability — specifically, a reduction in the threshold for action potential generation in peripheral motor neurons and sensory fibers. The most common mechanism is hypocalcemia: ionized calcium stabilizes neuronal membranes by reducing sodium permeability; when ionized calcium falls, sodium channels become abnormally permeable at resting membrane potential, causing spontaneous and repetitive neuronal firing.
The same physiological mechanism is activated by:
- Hypomagnesemia — magnesium is required for PTH secretion and peripheral PTH receptor action; hypomagnesemia produces functional hypoparathyroidism and secondary hypocalcemia
- Metabolic or respiratory alkalosis — alkalotic pH reduces the ionized fraction of total serum calcium (without changing total calcium) because hydrogen ions compete with calcium for albumin binding sites; hyperventilation-induced respiratory alkalosis is a classic precipitant of tetany in anxious patients
- Hyperventilation — acute respiratory alkalosis; may produce tetany even with normal total serum calcium because ionized calcium drops
- Hypokalemia — severe potassium depletion can also increase neuromuscular excitability, though less commonly causes classic tetany
- Hypoparathyroidism (Excludes 1 — if confirmed, assign E20.x)
Clinical Presentation
Patients coded to R29.0 exhibit the classic tetany syndrome:
- Carpopedal spasm — involuntary flexion of wrist and metacarpophalangeal joints with extension of interphalangeal joints; foot plantar flexion with toe extension; ICD-10-CM includes this as a term under R29.0
- Chvostek’s sign — ipsilateral facial muscle contraction with tapping over the facial nerve (preauricular area)
- Trousseau’s sign — carpopedal spasm induced by 3 minutes of brachial artery occlusion above systolic; highly specific for hypocalcemia
- Paresthesias — circumoral numbness, fingertip tingling, and perioral paresthesias often precede overt spasm
- Muscle cramps — generalized; may be the presenting complaint before full carpopedal spasm develops
- Laryngospasm — stridor, hoarseness, or respiratory distress in severe hypocalcemia; airway emergency
- Seizure-like activity — in severe hypocalcemic tetany; must distinguish from true epilepsy
Etiology
| Underlying Cause | ICD-10-CM Code | Sequencing Note |
|---|---|---|
| Hypocalcemia, unspecified | E83.51 | May code alongside R29.0; sequence per reason for encounter — see note below |
| Hypomagnesemia | E83.42 | Code alongside R29.0; hypomagnesemia → secondary hypocalcemia → tetany |
| Metabolic alkalosis | E87.3 | Code alongside R29.0 when alkalosis is confirmed driver |
| Respiratory alkalosis / hyperventilation | R06.4 (hyperventilation) + E87.3 | Acute respiratory alkalosis from hyperventilation — code the cause |
| Vitamin D deficiency | E55.9 | Causes hypocalcemia → tetany; sequence per reason for encounter |
| Chronic kidney disease (CKD-related hypocalcemia) | N18.x | CKD drives calcium/phosphate dysregulation; code N18.x + E83.51 + R29.0 |
| Hypoparathyroidism (non-surgical) | E20.9 or specific E20.x | Excludes 1 — when parathyroid failure confirmed, do NOT use R29.0 |
| Post-thyroidectomy / post-parathyroidectomy | E89.2 | Excludes 1 — do NOT use R29.0 for post-surgical parathyroid failure |
| Neonatal tetany (no Ca/Mg deficiency) | P71.3 | Excludes 1 — neonatal presentation; do NOT use R29.0 |
| Hysterical / functional tetany | F44.5 | Excludes 1 — psychiatric/conversion etiology; do NOT use R29.0 |
| Idiopathic — workup negative | R29.0 alone | R29.0 as sole code is appropriate when no etiology is established |
Sequencing — When Etiology Is Identified, It Typically Sequences First
When a specific metabolic etiology for tetany is confirmed (e.g., hypocalcemia, hypomagnesemia, vitamin D deficiency), the etiology code typically sequences as principal diagnosis and R29.0 becomes an additional code — per ICD-10-CM guidance that the condition established after study is the principal diagnosis, not the sign/symptom that prompted the encounter. If the tetany itself was the reason for admission and the workup remains inconclusive at discharge, R29.0 may sequence as principal.
Documentation Requirements
For accurate assignment of R29.0 and to support upgrade to a definitive etiology code:
- Clinical documentation of tetany — physician documentation of “tetany,” “carpopedal spasm,” Chvostek’s or Trousseau’s sign, or equivalent neuromuscular hyperexcitability findings
- Excludes 1 review — confirm no post-surgical parathyroid failure (E89.2), no confirmed hypoparathyroidism (E20.x), not a neonatal patient (P71.3), and not a conversion disorder (F44.5)
- Electrolyte and metabolic workup — serum calcium (total and ionized), magnesium, phosphate, PTH, pH, albumin documented
- Etiology statement — if the physician identifies the metabolic cause, document and code it alongside R29.0; if unresolved, R29.0 stands as the finding code
- Severity indicators — laryngospasm, seizure-like activity, airway compromise support MCC-tier additional diagnoses
💰 HCC Risk Adjustment (CMS-HCC v28)
| Field | Detail |
|---|---|
| CMS-HCC Model Version | v28 (2024-2025 Implementation) |
| HCC Assignment | ❌ Not Mapped |
| HCC Category | N/A |
| RAF Coefficient | 0.000 |
| RxHCC Assignment | Not Mapped |
R29.0 does not map to a CMS-HCC category under v28 and does not contribute to a patient’s Risk Adjustment Factor (RAF) score.
Monitor for RAF-Bearing Underlying Etiologies
While R29.0 itself carries no HCC weight, conditions frequently causing tetany do carry risk adjustment significance. At every R29.0 encounter, review and ensure complete coding of:
- Chronic kidney disease (N18.3-N18.6) — CMS-HCC v28 HCC-mapped (CKD 3-5 and ESRD mapped to HCC 329/330/331)
- Hypoparathyroidism — when confirmed, use E20.x (Excludes 1); review HCC mapping of specific E20 code assigned
- Vitamin D deficiency (E55.x) — review HCC mapping for v28
- Malnutrition (E43, E44.x) — HCC-mapped; may coexist and drive calcium/magnesium deficiency
Do not leave risk-adjustable comorbidities undercoded.
🏥 MS-DRG Assignment
MDC 01 — Diseases and Disorders of the Nervous System
| DRG | Title | Est. Relative Weight* |
|---|---|---|
| DRG 091 | Other Disorders of Nervous System with MCC | ~1.40-1.80 |
| DRG 092 | Other Disorders of Nervous System with CC | ~0.90-1.20 |
| DRG 093 | Other Disorders of Nervous System without CC/MCC | ~0.65-0.85 |
*Approximate. Verify against IPPS FY2026 Final Rule tables.
R29.0 as Principal Often Shifts MDC When Etiology Is Found
When the metabolic etiology driving tetany is identified during the admission workup — hypocalcemia (E83.51), hypomagnesemia (E83.42), vitamin D deficiency (E55.x) — and that condition sequences as principal, the encounter may move from MDC 01 to MDC 10 (Endocrine, Nutritional, and Metabolic Diseases), grouping to DRG 640-641-642 range instead of DRG 091-093. Always confirm principal diagnosis before accepting the DRG assignment.
🔗 Related ICD-10-CM Codes
R29 Category Sibling Codes
| Code | Description |
|---|---|
| R29.0 | Tetany ← This Code |
| R29.1 | Meningismus |
| R29.2 | Abnormal reflex |
| R29.3 | Abnormal posture |
| R29.5 | Transient paralysis |
| R29.6 | Repeated falls |
Excludes 1 Codes — Never Code Simultaneously with R29.0
| Code | Description | Why Excluded |
|---|---|---|
| F44.5 | Conversion disorder with seizures or convulsions | Hysterical tetany — psychiatric/functional etiology |
| P71.3 | Neonatal tetany without Ca/Mg deficiency | Neonatal-specific perinatal code |
| E20.9 | Hypoparathyroidism, unspecified | Parathyroid tetany — definitive endocrine code |
| E89.2 | Post-procedural hypoparathyroidism | Post-thyroidectomy/parathyroidectomy tetany |
Commonly Associated Additional Diagnosis Codes
| Code | Description | Coding Relevance |
|---|---|---|
| E83.51 | Hypocalcemia | Most common metabolic cause of tetany; NOT Excludes 1 — code alongside R29.0 |
| E83.42 | Hypomagnesemia | Second most common electrolyte cause; code alongside R29.0 |
| E87.3 | Alkalosis | Respiratory or metabolic alkalosis reduces ionized calcium; code alongside R29.0 |
| R06.4 | Hyperventilation | Precipitates respiratory alkalosis — tetany from anxiety/hyperventilation |
| E55.9 | Vitamin D deficiency, unspecified | Chronic cause of hypocalcemia — code alongside E83.51 and R29.0 |
| N18.4 | Chronic kidney disease, stage 4 | CKD drives calcium dysregulation; HCC-mapped in v28 |
| N18.5 | Chronic kidney disease, stage 5 | Advanced CKD — highest calcium disruption risk; HCC-mapped |
| E20.0 | Idiopathic hypoparathyroidism | Excludes 1 — if confirmed, replace R29.0 |
| E20.1 | Pseudohypoparathyroidism | Excludes 1 path — confirm whether R29.0 applies |
| M62.838 | Other muscle spasm | May co-occur with tetany when muscle spasm is separately documented at a specific site |
| R25.2 | Cramp and spasm | Symptom of localized cramp — distinct from tetany syndrome; not interchangeable |
| J38.5 | Laryngeal spasm | Laryngospasm as a severe complication of hypocalcemic tetany — code additionally when documented |
🛠️ Commonly Associated CPT Codes
Outpatient and Physician Setting Context
The CPT codes below are associated with the evaluation and management of tetany, electrolyte replacement, and related diagnostic workup in outpatient, ED, and physician fee schedule settings. In the inpatient setting, ICD-10-PCS procedure codes govern procedural reporting.
| CPT Code | Description | Clinical Application |
|---|---|---|
| 99285 | Emergency department E/M, high complexity | Initial ED evaluation of acute tetany with carpopedal spasm |
| 99214 | Office visit, established patient, moderate complexity | Follow-up for managed electrolyte-driven tetany |
| 99215 | Office visit, established patient, high complexity | Complex endocrine/renal follow-up with tetany workup |
| 96365 | IV infusion, initial up to 1 hour, non-chemotherapy | IV calcium gluconate or calcium chloride for acute hypocalcemic tetany |
| 96366 | IV infusion, each additional hour | Additional infusion hour for continued calcium or magnesium replacement |
| 96374 | IV push, single or initial substance | IV calcium push (urgent setting — laryngospasm risk) |
| 82310 | Calcium, total, serum | Serum total calcium — primary lab for tetany workup |
| 82330 | Calcium, ionized | Ionized calcium — more clinically relevant than total calcium in acid-base disturbance |
| 83735 | Magnesium, serum | Serum magnesium — essential in tetany workup |
| 84105 | Phosphorus, serum | Calcium-phosphate inverse relationship — important in renal and parathyroid workup |
| 83970 | Parathyroid hormone (PTH), intact | PTH level — distinguishes hypoparathyroidism from other tetany causes |
| 95886 | Needle EMG, each extremity, complete study | Rarely needed; used to characterize neuromuscular excitability pattern when etiology unclear |
NCCI Bundling Considerations
NCCI PTP Edits — Verify Before Billing
- 96365 (IV infusion) billed same DOS as E/M: Modifier -25 required on the E/M when separately documented beyond the pre/post infusion assessment.
- 82310 (total calcium) and 82330 (ionized calcium) billed same DOS: both are separately payable with distinct clinical indications; confirm each has medical necessity documentation.
- 83735 (magnesium) and 84105 (phosphorus) same DOS as calcium labs: typically separately payable as part of a comprehensive metabolic evaluation; verify LCD requirements for medical necessity.
🔬 ICD-10-PCS Crosswalk (Inpatient Procedures)
When R29.0 is an inpatient diagnosis and a procedure is performed, the following ICD-10-PCS sections and root operations are relevant. Full PCS codes require completion of all seven characters — consult the PCS tables for the applicable fiscal year.
| PCS Section | Body System | Root Operation | Clinical Application |
|---|---|---|---|
| 3 (Administration) | 3 (Peripheral Vein) | 0 (Introduction) | IV calcium gluconate/chloride infusion — Introduction of Electrolytic Substance, Peripheral Vein, Percutaneous |
| 3 (Administration) | 3 (Peripheral Vein) | 0 (Introduction) | IV magnesium sulfate infusion — Introduction of Electrolytic Substance, Peripheral Vein, Percutaneous |
| 4 (Measurement and Monitoring) | A (Physiological Systems) | 1 (Monitoring) | Continuous cardiac monitoring (QTc prolongation in hypocalcemia) |
| 3 (Administration) | 3 (Peripheral Vein) | 0 (Introduction) | IV Vitamin D / calcitriol administration when IV route used |
| B (Imaging) | 2 (Heart) | 3 (Fluoroscopy) | Not typical; cardiac telemetry crosswalk captured under monitoring above |
💊 Coding Scenarios and Examples
Scenario 1 — Acute Tetany, Hypocalcemia Identified, Etiology Unknown (ED / Inpatient)
Clinical Vignette: A 38-year-old female presents to the ED with acute carpopedal spasm, circumoral numbness, and positive Trousseau’s sign. No prior thyroid or parathyroid surgery. Labs: total serum calcium 6.8 mg/dL (critically low), ionized calcium 0.72 mmol/L, PTH pending. Admitted for IV calcium gluconate infusion and workup. Physician documents: “Acute tetany secondary to hypocalcemia — etiology under investigation.” PTH returns low-normal; vitamin D level low. Discharge diagnosis: “Hypocalcemia with tetany; vitamin D deficiency.”
Principal Diagnosis:
- E83.51 — Hypocalcemia (confirmed etiology established during admission — sequences as principal per coding guidelines)
Additional Diagnoses:
- R29.0 — Tetany (the clinical manifestation — code additionally when documented; hypocalcemia is NOT Excludes 1)
- E55.9 — Vitamin D deficiency, unspecified (contributing cause of hypocalcemia — code additionally)
MS-DRG Assignment:
- Hypocalcemia as principal → MDC 10 (Endocrine/Metabolic) — DRG 640/641/642 (not MDC 01); R29.0 as additional does not shift MDC
Hypocalcemia Is NOT Excludes 1 — Code Both
Scenario 2 — Post-Thyroidectomy Tetany — Excludes 1 Applies (Inpatient)
Clinical Vignette: A 47-year-old female undergoes total thyroidectomy for papillary thyroid carcinoma. On post-op day 1, she develops perioral tingling, positive Chvostek’s sign, and mild carpopedal spasm. Serum calcium 7.1 mg/dL. Endocrine consult documents: “Post-thyroidectomy hypoparathyroidism — tetany. IV calcium gluconate initiated.”
Correct Coding:
- E89.2 — Post-procedural hypoparathyroidism (Excludes 1 — this is the definitive code for post-surgical parathyroid failure; replaces R29.0 entirely)
- E83.51 — Hypocalcemia (may code additionally — E83.51 is not Excludes 1 to E89.2; code the resulting hypocalcemia separately)
Do NOT Assign:
R29.0— Excludes 1 — E89.2 is the correct code for post-thyroidectomy tetany; R29.0 is mutually exclusive with E89.2
E89.2 Is the Most Common R29.0 Excludes 1 Violation in Inpatient Coding
Post-thyroidectomy and post-parathyroidectomy encounters are the highest-risk setting for incorrectly assigning R29.0. The surgical history must be reviewed before accepting R29.0. If any prior thyroid or parathyroid surgery is documented, evaluate whether E89.2 is the correct code. Assigning R29.0 on these encounters is an Excludes 1 violation.
Scenario 3 — Tetany from Hyperventilation, No Electrolyte Abnormality (ED)
Clinical Vignette: A 22-year-old female presents to the ED after a panic attack with hyperventilation. She developed bilateral hand cramping with wrist flexion and finger extension (carpopedal spasm) and perioral tingling. ABG confirms acute respiratory alkalosis (pH 7.54, pCO2 28). Total calcium 9.1 mg/dL (normal), ionized calcium low-normal at 0.96. Physician documents: “Tetany secondary to hyperventilation-induced respiratory alkalosis. No structural hypocalcemia.” Resolves with rebreathing and anxiolytic.
ICD-10-CM (ED — First-Listed):
- R29.0 — Tetany (carpopedal spasm from respiratory alkalosis — no Excludes 1 condition present; R29.0 is appropriate)
- E87.3 — Alkalosis (documented respiratory alkalosis driving ionized calcium reduction — code additionally)
- R06.4 — Hyperventilation (precipitating cause — code additionally)
- F41.0 — Panic disorder with agoraphobia (or F41.1 generalized anxiety — if physician establishes psychiatric diagnosis; code additionally)
Normal Total Calcium Does Not Exclude Tetany or R29.0
Hyperventilation-induced tetany occurs with a normal total serum calcium because alkalotic pH shifts the calcium-albumin binding equilibrium, reducing the ionized fraction without changing total calcium. R29.0 is clinically and coding-appropriate in this scenario. The lack of true hypocalcemia (E83.51) does not mean the tetany coding is incorrect — it means E83.51 is not separately coded.
Scenario 4 — Tetany, Hypomagnesemia-Driven (Inpatient)
Clinical Vignette: A 61-year-old male with a history of alcohol use disorder is admitted with weakness, carpopedal spasm, and positive Chvostek’s sign. Labs: serum magnesium 0.8 mg/dL (critically low), serum calcium 7.6 mg/dL (low). Physician documents: “Tetany secondary to hypomagnesemia — likely from chronic alcohol use and poor nutrition. IV magnesium and calcium replacement initiated.”
Principal Diagnosis:
- E83.42 — Hypomagnesemia (confirmed etiology driving admission — sequences as principal)
Additional Diagnoses:
- R29.0 — Tetany (documented clinical manifestation — code additionally; hypomagnesemia is NOT Excludes 1)
- E83.51 — Hypocalcemia (secondary hypocalcemia from hypomagnesemia — code additionally when documented)
- F10.20 — Alcohol use disorder, uncomplicated (or more specific F10.x code per severity — underlying cause of hypomagnesemia)
Hypomagnesemia → Secondary Hypocalcemia → Tetany — Code All Three
Hypomagnesemia impairs both PTH secretion and PTH receptor action, producing a secondary functional hypocalcemia. When the physician documents this chain, code all three — E83.42, E83.51, and R29.0 — as each adds clinical specificity and may independently affect DRG tier and medical necessity documentation.
⚠️ Coding Pitfalls and Tips
| Pitfall or Tip | |
|---|---|
| ❌ | Do not submit R29 alone (3 characters) — non-billable parent; always submit R29.0 |
| ❌ | Do not use R29.0 when post-thyroidectomy/parathyroidectomy hypoparathyroidism is present — Excludes 1; assign E89.2 |
| ❌ | Do not use R29.0 when hypoparathyroidism is confirmed as the etiology — Excludes 1; assign E20.x |
| ❌ | Do not use R29.0 for neonatal patients — Excludes 1; assign P71.3 for neonatal tetany without Ca/Mg deficiency |
| ❌ | Do not use R29.0 for hysterical/conversion tetany — Excludes 1; assign F44.5 |
| ❌ | Do not confuse R29.0 with R25.2 (cramp and spasm) — tetany is a systemic neuromuscular hyperexcitability syndrome; localized muscle cramping maps to R25.2 or M62.83x |
| ❌ | Do not default R29.0 to MDC 01/DRG 091-093 when etiology is established — confirmed metabolic etiology as principal may shift the encounter to MDC 10 |
| ✅ | Hypocalcemia (E83.51) is NOT Excludes 1 — code both when tetany and hypocalcemia are both documented; this is the most common correct dual-coding scenario |
| ✅ | Hypomagnesemia (E83.42) is NOT Excludes 1 — code alongside R29.0 when both are documented |
| ✅ | Alkalosis (E87.3) is NOT Excludes 1 — code alongside R29.0 for hyperventilation-induced or metabolic alkalosis tetany |
| ✅ | Always review surgical history before assigning R29.0 — prior thyroidectomy or parathyroidectomy should immediately trigger consideration of E89.2 |
| ✅ | Sequence the etiology as principal when identified — R29.0 as additional code; if tetany remains the only documented finding at discharge, R29.0 as principal is appropriate |
| ✅ | Laryngospasm (J38.5) as complication of tetany is separately codeable — document and code when physician documents laryngeal involvement; may affect severity tier |
| ✅ | Sweep for RAF-bearing comorbidities — CKD (N18.x), malnutrition (E43), and hypoparathyroidism variants that fall outside Excludes 1 carry HCC weight that R29.0 does not |
📚 Sources
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CMS/NCHS. ICD-10-CM Official Guidelines for Coding and Reporting, FY2026. Tabular List — R29.0; R29 category notes; Excludes 1 notations (F44.5, P71.3, E20.9, E89.2); Chapter 18 signs and symptoms guidelines.
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AMA. CPT Professional Edition 2026. Evaluation and Management guidelines; Medicine — IV Infusion subsection (96360-96379); Pathology and Laboratory — Chemistry subsection.
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CMS. 2025-2026 Medicare Advantage Risk Adjustment — CMS-HCC Model v28 ICD-10-CM Mappings. R29.0 HCC mapping verification — no direct HCC assignment confirmed.
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CMS. IPPS Final Rule FY2026 — MS-DRG Definitions Manual v43. MDC 01 logic tables — Other Disorders of Nervous System DRG grouping (DRG 091-093); MDC 10 endocrine/metabolic DRG grouping cross-reference.
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CMS. ICD-10-PCS Reference Manual FY2026. Section 3 (Administration), Body System 3 (Peripheral Vein); Section 4 (Measurement and Monitoring).
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Cooper MS, Gittoes NJ. Diagnosis and management of hypocalcaemia. BMJ. 2008;336(7656):1298-1302. (Clinical basis for tetany presentation, Chvostek’s/Trousseau’s signs, and metabolic etiology framework reflected in R29.0 coding scenarios.)
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AMA. CPT Professional Edition 2026. Pathology and Laboratory — Chemistry subsection (82310, 82330, 83735, 83970).
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CMS. NCCI Policy Manual for Medicare Services, current version. General correct coding principles; IV infusion/injection bundling guidelines.