Understanding Raised Haemoglobin in Dehydration

The patient presents with symptoms of significant fluid loss: severe diarrhoea and vomiting. This leads to dehydration and hypotension (low blood pressure).

Physiological Impact of Dehydration

Dehydration causes a decrease in the plasma volume within the blood. Haemoglobin concentration is measured as the amount of haemoglobin per unit of blood volume.

• When the plasma volume decreases, the concentration of red blood cells, including haemoglobin, increases relative to the total blood volume.
• This makes the measured haemoglobin level appear higher than the patient's actual red blood cell mass.

Identifying Relative Polycythaemia

A haemoglobin level of $19$ gm% in this context is most likely due to haemoconcentration caused by dehydration.

• Relative polycythaemia is characterized by an increase in haemoglobin concentration due solely to a decrease in plasma volume, without an actual increase in red blood cell production.
• The patient's clinical presentation (hypotension, dehydration) strongly supports this diagnosis.

Differentiating Other Conditions

• Secondary polycythaemia involves increased red blood cell production due to factors like hypoxia or tumours, which is less likely given the acute symptoms.
• Polycythaemia vera is a bone marrow disorder causing overproduction of red blood cells, typically presenting more chronically.
• Essential thrombocythaemia involves increased platelet production, not primarily haemoglobin.

Therefore, the most likely reason for the raised haemoglobin is relative polycythaemia resulting from dehydration.