# Situation: A 76-year-old woman with type 2 diabetes mellitus is brought from home after three days of increasing drowsiness and poor oral intake during a febrile illness. She weighs 60 kg. The provider diagnoses hyperosmolar hyperglycemic state (HHS). Her results are sodium 141 mEq/L, glucose 1,026 mg/dL (57.0 mmol/L), and blood urea nitrogen (BUN) 42 mg/dL (15.0 mmol/L). Total serum osmolality = 2 × sodium + glucose (mg/dL) ÷ 18 + BUN (mg/dL) ÷ 2.8. Effective osmolality leaves out any solute that crosses cell membranes freely. What is her effective serum osmolality? Round off to the nearest whole number.

> source: MyMerci (mymerci.kr)  
> url: https://mymerci.kr/pages/nclex_q.php?qn_id=630227  
> language: ko  
> subject: Nursing Practice V — Care of Clients with Maladaptive Patterns of Behavior; Care of Clients with Life-Threatening Conditions, Acute Multi-Organ Problems, High Acuity and Emergency Situations

## 문제

Situation: A 76-year-old woman with type 2 diabetes mellitus is brought from home after three days of increasing drowsiness and poor oral intake during a febrile illness. She weighs 60 kg. The provider diagnoses hyperosmolar hyperglycemic state (HHS).

Her results are sodium 141 mEq/L, glucose 1,026 mg/dL (57.0 mmol/L), and blood urea nitrogen (BUN) 42 mg/dL (15.0 mmol/L). Total serum osmolality = 2 × sodium + glucose (mg/dL) ÷ 18 + BUN (mg/dL) ÷ 2.8. Effective osmolality leaves out any solute that crosses cell membranes freely. What is her effective serum osmolality? Round off to the nearest whole number.

## 보기

1. 354 mOsm/kg
2. 385 mOsm/kg
3. 339 mOsm/kg **✔ 정답**
4. 198 mOsm/kg

**정답: 3**

## 해설

Urea crosses cell membranes freely, so it does not pull water out of cells and is left out of effective osmolality: 2 × 141 + 1,026 ÷ 18 = 282 + 57 = 339 mOsm/kg. This is well above the HHS criterion of more than 300 mOsm/kg.

## 심화 해설

Understanding effective osmolality in HHS

The key to this calculation is distinguishing total serum osmolality from effective serum osmolality. Both are measures of solute concentration, but they differ in one critical way: whether the solute can cross cell membranes.

Urea crosses cell membranes freely, so it distributes evenly between the intracellular and extracellular compartments and does not create an osmotic gradient that pulls water out of cells. Because urea does not contribute to water shifts, it is excluded when calculating effective osmolality. Sodium and glucose, in contrast, are effective osmoles because they remain primarily in the extracellular space and do create osmotic gradients.

The formula for effective osmolality is:

Effective osmolality = 2 × sodium + glucose (mg/dL) ÷ 18

Plugging in the patient's values:

- Sodium = 141 mEq/L, so 2 × 141 = 282

- Glucose = 1,026 mg/dL, so 1,026 ÷ 18 = 57

- Effective osmolality = 282 + 57 = 339 mOsm/kg

The BUN of 42 mg/dL is intentionally omitted. If it were included, the total osmolality would be 339 + (42 ÷ 2.8) = 339 + 15 = 354 mOsm/kg, which corresponds to option 1. Watch out! Option 1 is the total osmolality, not the effective osmolality. The question specifically asks for effective osmolality, so urea must be left out.

An effective osmolality of 339 mOsm/kg is well above the HHS diagnostic threshold of greater than 300 mOsm/kg, confirming the hyperosmolar state. This elevation is driven almost entirely by the marked hyperglycemia, which pulls water from the intracellular space into the extracellular compartment, diluting the sodium concentration. The measured sodium of 141 mEq/L may appear normal or even low-normal, but it does not reflect the true total body sodium deficit because of the dilutional effect of glucose-driven water shifts.

Key point! In hyperglycemic emergencies, the presenting sodium concentration underestimates the actual sodium deficit. The effective osmolality calculation helps quantify the severity of the hyperosmolar state and guides the rate and composition of fluid replacement. The tonicity disorder in HHS is hypertonic, meaning water moves out of cells, causing intracellular dehydration despite the patient's overall volume depletion.

The distinction between total and effective osmolality also matters when interpreting sodium values in other settings. A sodium concentration that appears normal or low in the presence of severe hyperglycemia or marked azotemia may not indicate true hypotonicity, because glucose and urea affect osmolality differently. Glucose is an effective osmole and raises tonicity, while urea is an ineffective osmole and does not alter tonicity even when BUN is elevated.

## 임상 시나리오

HHS Effective OsmolalityWhy urea is excluded from the calculation
In hyperosmolar hyperglycemic state, calculate effective osmolality as 2 × sodium + glucose/18. For this patient, 2 × 141 + 1026/18 = 339 mOsm/kg, which exceeds the HHS threshold of >300 mOsm/kg.

Urea crosses cell membranes freely and does not create an osmotic gradient, so BUN is omitted from effective osmolality. Including BUN gives total osmolality of 354 mOsm/kg, which is not the value used to assess cellular dehydration.

CautionDo not use total osmolality to define HHS. Adding BUN overestimates the osmotic force driving water out of cells and may delay recognition of the hyperosmolar emergency.

## 핵심 개념

- **Effective osmolality** — Solute concentration that excludes freely membrane-permeable solutes such as urea; reflects osmoles capable of creating water shifts.
- **Hyperosmolar hyperglycemic state** — Severe hyperglycemic emergency with marked hyperosmolality, dehydration, and altered mental status without significant ketoacidosis.
- **Urea** — A nitrogenous waste product that crosses cell membranes freely and therefore does not contribute to effective osmolality.
- **Total serum osmolality** — Calculated as 2×Na + glucose/18 + BUN/2.8; includes all major solutes regardless of membrane permeability.
- **Osmotic gradient** — Difference in solute concentration across a semipermeable membrane that drives water movement between compartments.

## 같은 주제 문제

- [Situation: A 58-year-old man with severe community-acquired pneumonia is intubated in the …](https://mymerci.kr/pages/nclex_q.php?qn_id=630016)
- [Situation: A 58-year-old man with severe community-acquired pneumonia is intubated in the …](https://mymerci.kr/pages/nclex_q.php?qn_id=630017)
- [Situation: A 58-year-old man with severe community-acquired pneumonia is intubated in the …](https://mymerci.kr/pages/nclex_q.php?qn_id=630018)
- [Situation: A 58-year-old man with severe community-acquired pneumonia is intubated in the …](https://mymerci.kr/pages/nclex_q.php?qn_id=630019)
- [Situation: A 62-year-old man arrives at the emergency department of a provincial hospital …](https://mymerci.kr/pages/nclex_q.php?qn_id=630020)
- [Situation: A 62-year-old man arrives at the emergency department of a provincial hospital …](https://mymerci.kr/pages/nclex_q.php?qn_id=630021)
- [Situation: A 62-year-old man arrives at the emergency department of a provincial hospital …](https://mymerci.kr/pages/nclex_q.php?qn_id=630022)
- [Situation: A 70-year-old woman with rheumatic heart disease is in the coronary care unit. …](https://mymerci.kr/pages/nclex_q.php?qn_id=630023)

---

More free questions: [기출문제](https://mymerci.kr/)

_학습 참고용입니다. 실제 임상은 최신 지침과 소속 기관 프로토콜을 따르세요._