# Situation: A 40-year-old man with dengue is admitted on the fourth day of illness; his fever subsided this morning. He weighs 60 kg and has no history of heart or kidney disease. He is receiving intravenous (IV) fluids through a 20-gauge catheter in his left forearm. His results over 2 days are: Day 3: hematocrit 42%, platelet count 110,000/mm³ Day 4: hematocrit 51%, platelet count 38,000/mm³ He has been drinking well, his IV fluids have run as ordered, and he has no bleeding. Which process do these results MOST likely indicate?

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## 문제

Situation: A 40-year-old man with dengue is admitted on the fourth day of illness; his fever subsided this morning. He weighs 60 kg and has no history of heart or kidney disease. He is receiving intravenous (IV) fluids through a 20-gauge catheter in his left forearm.

His results over 2 days are:
Day 3: hematocrit 42%, platelet count 110,000/mm³
Day 4: hematocrit 51%, platelet count 38,000/mm³
He has been drinking well, his IV fluids have run as ordered, and he has no bleeding. Which process do these results MOST likely indicate?

## 보기

1. Increased red cell production as the bone marrow recovers
2. Plasma leaking from the vessels into the tissue spaces **✔ 정답**
3. Hemoconcentration from inadequate oral and IV fluid intake
4. Hidden internal bleeding that is using up his platelets

**정답: 2**

## 해설

In the critical phase of dengue, around the time the fever subsides, capillaries leak plasma into the tissues. A rising hematocrit with a rapid fall in platelets is the warning sign of plasma leakage. His intake has been adequate, so dehydration does not explain the rise, and bleeding would lower the hematocrit.

## 심화 해설

Clinical situation

A 40-year-old man with dengue is on day 4 of illness, and his fever subsided this morning. His hematocrit rose from 42% on day 3 to 51% on day 4, while his platelet count dropped from 110,000/mm³ to 38,000/mm³. He is drinking well, IV fluids have run as ordered, and he has no bleeding.

Answer and core mechanism

The most likely process is plasma leakage from the intravascular space into the interstitial and serosal compartments. This is the hallmark of the critical phase of dengue, which typically begins around defervescence—exactly when this patient’s fever subsided. A rising hematocrit combined with a rapidly falling platelet count is the classic warning sign of plasma leakage, not of bleeding or dehydration.

Why the hematocrit rises

When plasma leaks out of capillaries, the liquid portion of blood leaves the vessel while red blood cells remain behind. The same number of red cells is now suspended in a smaller plasma volume, so the measured hematocrit rises. This is hemoconcentration. In dengue, a hematocrit increase of 20% or more from baseline is a key indicator for intervention [3]. Here the hematocrit rose from 42% to 51%, which is approximately a 21% relative increase—crossing that threshold.

Why the platelets fall

Thrombocytopenia in dengue results from both peripheral destruction and bone marrow suppression. During the critical phase, platelets are consumed and destroyed rapidly, often dropping sharply over 24–48 hours. The fall from 110,000/mm³ to 38,000/mm³ in one day is a steep decline that parallels the onset of plasma leakage. The combination of hemoconcentration and rapid thrombocytopenia is far more specific for plasma leakage than for any single finding alone.

Ruling out the other options

| Option | Why it is incorrect |
| --- | --- |
| 1. Increased red cell production | Bone marrow recovery would take days to raise the hematocrit, not 24 hours. A rise this fast reflects fluid shifts, not new red cell synthesis. |
| 3. Hemoconcentration from inadequate intake | The patient has been drinking well and IV fluids have run as ordered. Dehydration would also typically show other signs, and the platelet drop would not be explained by poor intake alone. |
| 4. Hidden internal bleeding | Bleeding loses whole blood—both plasma and red cells—so the hematocrit would fall or remain stable, not rise to 51%. The platelet drop here is due to dengue pathophysiology, not consumption by occult hemorrhage. |

Timing and clinical significance

Serial ultrasonography in dengue hemorrhagic fever shows that plasma leakage begins around the time of defervescence and can be detected as ascites, gallbladder wall thickening, and pleural effusions . The leakage is transient and typically resolves after 48–72 hours if the patient is supported with careful fluid management. Early recognition of this phase is critical because continued leakage can progress to dengue shock syndrome, characterized by narrowed pulse pressure and circulatory compromise .

Key point! The critical phase begins around defervescence, not at the peak of fever. A patient whose fever subsides may appear to be improving, but this is precisely when plasma leakage and shock can develop.

Watch out! A rising hematocrit in dengue is not a sign of adequate hydration or red cell recovery. It is a warning that plasma is leaving the vascular compartment and the patient may be heading toward hypovolemia despite normal fluid intake.

Bedside monitoring implications

Because hematocrit is an indirect marker of plasma leakage, point-of-care hemoglobin or hematocrit measurements can help detect hemoconcentration early in endemic settings . However, the trend matters more than a single value. A baseline hematocrit before the critical phase is essential for interpreting later changes. In this patient, the 9-point rise in one day is a strong signal that the critical phase has begun, and close monitoring of hemodynamic status, urine output, and signs of effusion is warranted.References (research sources)

- [3]In Silico Optimization of a Non-Invasive Optical Sensor for Hemoconcentration Monitoring in Dengue Fever Management.Research articleAlthobaiti M, Saleh G. (2026) · DOI: 10.3390/bios16020121

## 임상 시나리오

Dengue Critical Phase RecognitionPlasma Leakage at Defervescence
A rising hematocrit with a rapidly falling platelet count around the time fever subsides is the classic warning sign of plasma leakage in dengue. Hematocrit increase of 20% or more from baseline is a key intervention threshold.

Plasma leaks from capillaries into interstitial and serosal spaces while red blood cells remain intravascular, causing hemoconcentration. This patient's hematocrit rose from 42% to 51%, a relative increase of approximately 21%.

CautionDo not attribute a rising hematocrit to dehydration if intake has been adequate. Bleeding would lower the hematocrit, not raise it. Monitor for signs of shock and adjust IV fluid therapy according to dengue protocols.

## 핵심 개념

- **plasma leakage** — Escape of plasma from capillaries into interstitial or serosal spaces, the hallmark of dengue critical phase, causing hemoconcentration.
- **hemoconcentration** — Increased concentration of red blood cells in blood due to loss of plasma volume, reflected as a rising hematocrit.
- **critical phase** — Period in dengue, typically around defervescence (days 3-7), when plasma leakage and shock may occur.
- **defervescence** — The subsiding of fever; in dengue, this marks the transition into the critical phase when plasma leakage begins.
- **thrombocytopenia** — Low platelet count, in dengue caused by peripheral destruction and bone marrow suppression, often dropping sharply during the critical phase.

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