Hyponatremia workup algorithm

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hyponatremia workup diagnostic algorithm serum osmolality urine sodium 2024

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Hyponatremia Workup Algorithm

Hyponatremia = serum [Na+] < 135 mEq/L. It is among the most common electrolyte disorders encountered in hospitalized patients. The approach below follows a logical stepwise algorithm grounded in physiology.

Initial Labs to Order (All Patients)

Test	Purpose
Serum sodium, BMP	Confirm hyponatremia, assess K+, glucose, BUN/Cr
Serum osmolality	First branch point - classify type
Urine osmolality	Assess ADH activity / renal concentrating ability
Urine sodium (spot)	Assess renal sodium handling / volume status
TSH	Rule out hypothyroidism
Morning cortisol	Rule out adrenal insufficiency
Serum lipids, protein	If pseudohyponatremia suspected

Step 1 - Measure Serum Osmolality (First Branch Point)

Normal serum osmolality = 275-295 mOsm/kg

Serum Osmolality
├── Normal (275-295) or HIGH (>295)  →  NOT true hypotonic hyponatremia
│   ├── Normal Posm:  Pseudohyponatremia
│   │     • Marked hyperlipidemia
│   │     • Marked hyperproteinemia (e.g., multiple myeloma)
│   └── High Posm:  Translocational hyponatremia
│         • Hyperglycemia (Na drops ~1.6 mEq/L per 100 mg/dL glucose rise)
│         • Mannitol, sorbitol, glycine (urologic irrigants)
│
└── LOW (<275)  →  TRUE hypotonic hyponatremia  →  PROCEED TO STEP 2

Note: Hyperglycemia correction formula: corrected Na = measured Na + 1.6 × [(glucose - 100) / 100]

Step 2 - Search for Specific Diagnostic Clues (Quick Screen)

Before volume assessment, scan for highly specific findings (Figure 24-1, Symptom to Diagnosis 4e):

Hyponatremia Step 1 - Search for specific clues

Finding	Diagnosis to Consider
Thiazide use	Thiazide-induced hyponatremia
Urine Osm < 100 mOsm/kg	Water intoxication: psychogenic polydipsia, beer potomania, ecstasy, exercise-associated hyponatremia (EAH)
Marked hyperglycemia	Hyperglycemic-induced hyponatremia (translocational)
Markedly elevated creatinine	Kidney failure
Hyperkalemia + unexplained hypoglycemia	Primary adrenal insufficiency
Normal/elevated serum osmolality	Pseudohyponatremia
Recent marathon / endurance event	Exercise-associated hyponatremia
Recent "party"/rave	Ecstasy (MDMA) use

If none of the above clues are present - proceed to Step 3.

Step 3 - Clinical Volume Status Assessment

Assess volume status by physical examination (JVP, skin turgor, mucous membranes, edema, orthostatic BP):

TRUE Hypotonic Hyponatremia (Posm <275)
│
├── HYPOVOLEMIC (low volume)
│   Signs: Dry mucous membranes, decreased skin turgor,
│          tachycardia, orthostasis, low JVP
│
├── EUVOLEMIC (normal volume)
│   Signs: No edema, no orthostasis
│
└── HYPERVOLEMIC (excess volume)
    Signs: Edema, ascites, elevated JVP

Step 4 - Urine Sodium and Urine Osmolality (Refine the Differential)

Hypovolemic Hyponatremia (Volume-depleted)

Urine Na	Etiology
< 20-30 mEq/L (renal Na conserving)	Extra-renal losses: vomiting, diarrhea, sweating, third-spacing, burns
> 20-30 mEq/L (renal Na wasting)	Diuretics (thiazides > loop), salt-wasting nephropathy, cerebral salt wasting (CSW), primary adrenal insufficiency, mineralocorticoid deficiency

Key distinguisher: CSW vs SIADH - both have elevated urine Na and elevated urine Osm, but CSW is hypovolemic while SIADH is euvolemic.

Hypervolemic Hyponatremia (Edematous states)

Urine Na	Etiology
< 20 mEq/L (low effective arterial blood volume)	Heart failure, cirrhosis, nephrotic syndrome
> 20 mEq/L	Advanced renal failure (GFR < 5-10 mL/min)

Mechanism: In HF and cirrhosis, decreased effective circulating volume triggers ADH and promotes avid Na reabsorption - hence low UNa despite apparent volume overload.

Euvolemic Hyponatremia (Most common outpatient scenario)

This is the most complex category. Urine osmolality is the first branch:

Euvolemia
│
├── Urine Osm < 100 mOsm/kg  →  Maximally dilute urine, ADH suppressed
│     • Primary polydipsia (psychogenic)
│     • Beer potomania
│     • Ecstasy / MDMA
│     • Low-solute diet ("tea and toast")
│
└── Urine Osm > 100 mOsm/kg  →  Inappropriately concentrated, ADH active
      │
      ├── Check Urine Na
      │     • UNa < 20-30  →  Consider subtle hypovolemia
      │                        (re-classify as hypovolemic)
      │     • UNa > 30    →  Proceed to hormonal workup
      │
      └── Check TSH and Morning Cortisol  (Figure 24-4)

Finding	Diagnosis
TSH markedly elevated	Severe hypothyroidism
Morning cortisol < 16.3 mcg/dL	Evaluate for adrenal insufficiency (requires cosyntropin stimulation test to confirm)
Other etiologies excluded + Uosm elevated + UNa elevated	SIADH (diagnosis of exclusion)

SIADH Diagnostic Criteria (Tintinalli / classic criteria)

All of the following must be present:

Hypotonic hyponatremia (Posm < 275 mOsm/kg)
Urine osmolality inappropriately elevated (usually > 200, often > 100 mOsm/kg)
Urine Na > 20-40 mEq/L (ongoing renal Na loss)
Clinical euvolemia
Normal adrenal, renal, cardiac, hepatic, and thyroid function

SIADH causes: CNS disease (stroke, trauma, infection, mass), pulmonary disease (pneumonia, malignancy, respiratory failure), drugs (SSRIs, carbamazepine, cyclophosphamide, vincristine, NSAIDs, opioids), ectopic ADH production (small cell lung cancer, head/neck tumors, pancreas, lymphoma), HIV, postoperative state.

Summary Table: Urine Na + Urine Osm by Category

Volume Status	UNa	Urine Osm	Likely Etiology
Hypovolemic	< 20	> 100	GI/skin losses, third-spacing
Hypovolemic	> 20	> 100	Diuretics, salt-wasting nephropathy, adrenal insufficiency
Hypervolemic	< 20	> 100	HF, cirrhosis, nephrotic syndrome
Hypervolemic	> 20	Variable	Advanced renal failure
Euvolemic	< 20	< 100	Primary polydipsia, beer potomania
Euvolemic	> 40	> 100	SIADH, hypothyroidism, adrenal insufficiency

Rule of thumb (Tintinalli): UNa < 10 mEq/L is found only in edematous syndromes (HF, cirrhosis) and in vomiting/diarrhea.

Acute vs. Chronic Distinction (Critical for Treatment)

Acute: onset < 24-48 hours - higher risk of cerebral edema; more aggressive correction acceptable
Chronic: onset > 48 hours (or unknown) - assume chronic; risk of osmotic demyelination syndrome (ODS/CPM) with rapid correction
When duration is unknown, always assume chronic

Treatment Correction Rate Limits

Clinical Scenario	Max Correction Rate
Severe symptoms (seizures, obtundation, coma)	Raise by 4-6 mEq/L acutely with 3% NaCl bolus (100 mL over ~15 min), then reassess
Chronic / asymptomatic	≤ 6-8 mEq/L per 24 hours (high-risk patients)
Chronic / mild-moderate symptoms	≤ 10-12 mEq/L per 24 hours (low-risk patients)
48-hour limit	≤ 18 mEq/L total over 48 hours

High-risk for ODS: liver disease, alcoholism, malnutrition, severe hyponatremia (< 105), hypokalemia.

Sources: Symptom to Diagnosis, 4e (Figures 24-1, 24-4) | Tintinalli's Emergency Medicine (Table 17-6) | Harrison's Principles of Internal Medicine 22e | Brenner and Rector's The Kidney | ROSEN's Emergency Medicine

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Hyponatremia Quick Reference

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The quick reference PDF covers:

Step 0 - Initial labs checklist (serum Osm, urine Osm, urine Na, TSH, cortisol, etc.)
Step 1 - Serum osmolality branch point (pseudohyponatremia vs. translocational vs. true hypotonic)
Step 2 - Specific diagnostic clue screen (thiazides, Uosm < 100, hyperglycemia, hyperkalemia, party/Ecstasy, marathon, etc.)
Steps 3 & 4 - Three-column volume status layout (hypovolemic / euvolemic / hypervolemic) with UNa and Uosm cutoffs for each
SIADH criteria - All 5 diagnostic criteria + common causes organized in columns
Summary table - UNa + Uosm by volume category mapped to etiology
Correction limits table - Severe symptoms / acute / chronic low-risk / chronic high-risk scenarios with exact mEq/L/24h caps
ODS warning box - Red-bordered alert with rescue desmopressin + D5W protocol for overcorrection

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