Can dehydration mask anaemia in individuals with borderline haemoglobin (Hb) levels?

Can Dehydration Mask Anaemia in Borderline Haemoglobin Levels?

Yes, dehydration can mask anaemia by artificially elevating haemoglobin and haematocrit values through haemoconcentration, making borderline anaemia appear normal until rehydration occurs.

Mechanism of Haemoconcentration

When dehydration occurs, plasma volume contracts while red blood cell mass remains constant, leading to:

• Increased haemoglobin concentration due to reduced plasma volume rather than increased red cell mass 1
• Elevated haematocrit that falsely suggests adequate oxygen-carrying capacity 2
• Disproportionate elevation of BUN relative to creatinine (BUN:creatinine ratio >20:1), which helps identify the dehydrated state 1

The key physiological principle is that haemoglobin is measured as concentration (g/dL), not absolute mass. When the denominator (plasma volume) decreases, the concentration rises even though total haemoglobin remains unchanged 1.

Clinical Recognition of Masked Anaemia

To unmask true anaemia in dehydrated patients, you must:

• Measure serum osmolality directly (>300 mOsm/kg indicates dehydration requiring intervention) or calculate it using: osmolarity = 1.86 (Na+ + K+) + 1.15 × glucose + urea + 14, with threshold >295 mmol/L 3
• Check BUN:creatinine ratio - values exceeding 20:1 suggest volume depletion with urea reabsorption disproportionately increased compared to creatinine 1
• Recheck haemoglobin after adequate rehydration - the "true" haemoglobin will be revealed once plasma volume normalizes 2, 1

Critical Clinical Scenarios

This masking effect is particularly dangerous in:

• Patients with cyanotic congenital heart disease who require careful monitoring of both hydration status and haemoglobin, as dehydration must be avoided and therapeutic phlebotomy should only occur when haematocrit >65% in the absence of dehydration 2
• Surgical candidates where unrecognized anaemia combined with dehydration compounds oxygen delivery problems 2
• Patients with chronic severe anaemia who may have expanded plasma volumes at baseline (+70%), making dehydration-induced haemoconcentration bring haemoglobin into falsely "normal" range 4

Practical Diagnostic Approach

Follow this sequence when borderline haemoglobin is detected:

1. Assess hydration status immediately using serum osmolality (not clinical signs alone, which are unreliable especially in older adults) 3
2. Check BUN:creatinine ratio - if >20:1, suspect volume depletion 1
3. Ensure glucose and urea are normal before interpreting osmolality; if elevated, normalize these first 1, 3
4. Rehydrate appropriately:
   • For patients appearing well: encourage oral fluids 3
   • For patients appearing unwell: provide subcutaneous or intravenous fluids while encouraging oral intake 3
5. Recheck haemoglobin 24-48 hours after achieving euvolemia - monitor BUN:creatinine normalization and adequate urine output as markers of successful rehydration 1

Important Caveats

Do not rely on clinical examination alone to assess dehydration, particularly in older adults, as physical signs have poor sensitivity (41.1%) and modest specificity (80%) 2, 3.

In patients with poor renal function (common in elderly), renal parameters may not accurately signal dehydration, requiring direct osmolality measurement 3.

During rehydration, osmolality change should not exceed 3 mOsm/kg/h to avoid complications 1.

The European Society of Cardiology guidelines specifically note that in hyponatremic neurosurgical patients, those with severe dehydration on blood volume measurements were treatment non-responders, and patients with anaemia (hematocrit <27%) required whole blood in addition to saline 2. This underscores that dehydration and anaemia frequently coexist and must both be addressed.