Haemoglobin vs Haematocrit: Which Matters More on TRT?

Haemoglobin and haematocrit are the two most commonly used markers for monitoring polycythemia on TRT. Both rise with testosterone use and both are part of a standard complete blood count (CBC). Despite being closely related, they measure different things and behave differently under certain conditions.

Haemoglobin is the iron-containing protein inside red blood cells that binds oxygen. It is measured directly in grams per decilitre (g/dL) and reflects the blood's oxygen-carrying capacity.

Haematocrit is the percentage of total blood volume occupied by red blood cells. It is either calculated from other CBC parameters (MCV x RBC count) or measured directly by centrifugation. It reflects blood viscosity.

For TRT patients and bodybuilders, monitoring both is standard practice. Most clinical guidelines use haematocrit thresholds for intervention decisions (e.g., phlebotomy above 54%), but haemoglobin provides complementary information and is less susceptible to hydration-related errors.

What each measures:

• Haemoglobin: The concentration of the oxygen-carrying protein in blood. A direct, molecular measurement.
• Haematocrit: The volume fraction of blood occupied by red blood cells. A volumetric measurement.

Relationship:

• In health, haematocrit is approximately 3x haemoglobin (e.g., Hb 16 g/dL corresponds to Hct ~48%)
• This ratio can diverge with dehydration, large or small red cells (macrocytosis/microcytosis), or certain lab methods

Susceptibility to hydration:

• Haematocrit is significantly affected by hydration status. Dehydration concentrates red blood cells, falsely elevating haematocrit by 2-5 percentage points. This is the most common confounder in TRT monitoring.
• Haemoglobin is also affected by plasma volume but to a lesser degree, making it slightly more stable.

Measurement method:

• Haemoglobin is measured directly by spectrophotometry (lysing red cells and measuring the released haemoglobin). This is a precise, direct measurement.
• Haematocrit on modern analysers is typically calculated (MCV x RBC count / 10) rather than directly measured by centrifugation. Calculated values can have small systematic errors.

Clinical thresholds for TRT:

• Haematocrit: most guidelines recommend intervention above 54%. The Endocrine Society uses 54% as the action threshold.
• Haemoglobin: corresponding threshold is approximately 18-18.5 g/dL. Less standardised in TRT guidelines.

Iron deficiency effects:

• Iron-deficient erythropoiesis produces smaller red cells (microcytosis) with less haemoglobin per cell
• This can cause haemoglobin to drop while haematocrit remains relatively preserved (the cells are smaller but more numerous)
• Recognising this pattern is important for TRT patients who develop iron deficiency from phlebotomy

Use haematocrit for clinical decisions on TRT:

• This is the marker cited in TRT guidelines (Endocrine Society, AUA) for intervention thresholds
• Doctors and TRT clinics are most familiar with haematocrit-based decision-making
• The 54% threshold for phlebotomy is well-established

Use haemoglobin when you suspect hydration confounding:

• If haematocrit is borderline (52-55%) and you suspect dehydration from training, caffeine, or inadequate water intake, haemoglobin provides a cross-check
• A haematocrit of 55% with a haemoglobin of 17.5 g/dL (ratio ~3.14) suggests the haematocrit is falsely elevated by dehydration
• A haematocrit of 55% with a haemoglobin of 18.5 g/dL (ratio ~2.97) is more likely to be genuine polycythemia

Track both for the most reliable monitoring:

• Both are included in every CBC, so there is no additional cost
• Comparing the two catches measurement artifacts and hydration effects
• If one is elevated and the other is normal, investigate before intervening

Haemoglobin is more informative when:

• Assessing actual oxygen-carrying capacity and physiological risk
• Monitoring iron deficiency in TRT patients (haemoglobin drops before haematocrit)
• The patient has abnormal red cell size (MCV abnormalities)