Haematocrit, DVT risk, and blood donation on TRT

The single most reliable biochemical change on TRT is a rise in haematocrit, the proportion of blood volume made up of red cells. This is physiological and expected. Above a certain threshold it becomes a clinical problem. This article describes why haematocrit rises on TRT, what thresholds matter, and the practical interventions we use to manage it.

Why testosterone raises red cell count

Testosterone stimulates erythropoietin production and increases red cell precursor proliferation in bone marrow. Both effects combine to increase red cell mass. Blood volume expands slightly but less than the red cell mass increase, so haematocrit rises.

The effect is dose-dependent and consistent across individuals. Most men on standard TRT doses see a haematocrit rise of 3 to 5 percentage points over the first 6 to 12 months, then plateau. A smaller number see larger rises requiring intervention.

Injectable regimens with marked peak-trough variation (Nebido with long intervals, or weekly enanthate) tend to produce larger haematocrit rises than steady-state transdermal gels. Splitting injectable doses into smaller, more frequent injections reduces the effect.

The thresholds that matter

The normal male haematocrit range is roughly 0.40 to 0.52 depending on the lab. Above this:

- Haematocrit 0.52 to 0.54: cautious zone. Monitor closely. Consider dose reduction or injection-smoothing. - Haematocrit 0.54 to 0.56: act. Dose reduction, smoother dosing, hydration, investigate for untreated apnoea. - Haematocrit above 0.56: therapeutic venesection or voluntary blood donation. Dose review.

These are not arbitrary; they reflect the concentration above which blood viscosity increases meaningfully and thrombotic and cardiovascular event risk rises.

What happens when haematocrit is too high

Raised haematocrit (polycythaemia from TRT, or secondary erythrocytosis) increases blood viscosity. The consequences of uncorrected significant erythrocytosis include:

- Increased risk of venous thromboembolism (DVT and PE) - Increased risk of stroke - Increased risk of myocardial infarction - Impaired microcirculation, producing symptoms of headache, facial flushing, poor concentration, dizziness

These are not theoretical. Men on TRT who are not monitored and develop haematocrits of 0.58 to 0.60 are at genuine clinical risk.

How we monitor

Baseline haematocrit before starting TRT. Repeat at 6 to 8 weeks, 12 weeks, 6 months, 12 months. Then biannual.

If baseline is already in upper normal range (0.48 to 0.51), we start a lower dose of testosterone and monitor more frequently. Men who start with already-elevated haematocrit reliably have problems later unless managed proactively.

Interventions in order of preference

First: hydration. Some of the observed rise is apparent rise from mild dehydration at time of blood draw. Increasing daily fluid intake to 2.5 to 3 litres and fasting but well-hydrated for blood tests often drops the measured value by 2 to 3 points.

Second: dose smoothing. Convert Nebido at 12-week intervals to 10-week intervals with slightly smaller doses. Convert weekly enanthate to twice-weekly. This reduces the peak hormone level that drives erythrocytosis.

Third: dose reduction. Lower the testosterone dose slightly. For gels, drop from 60 to 50mg daily. For enanthate, drop weekly dose. This is usually well tolerated and haematocrit drops reliably within 6 to 12 weeks.

Fourth: check for sleep apnoea. Untreated obstructive sleep apnoea is a major amplifier of TRT-induced erythrocytosis. If apnoea is present and untreated, CPAP initiation often brings haematocrit down without any change to TRT. We arrange a WatchPAT study whenever haematocrit rises unexpectedly.

Fifth: therapeutic venesection. Removing 450 to 500 mL of blood (one unit) rapidly drops haematocrit by 1 to 2 percentage points. Can be done privately (roughly £100 to £200 per session) or, where eligible, as voluntary blood donation (free, and of social benefit). We arrange this when needed.

Blood donation considerations

Men on TRT are eligible to donate blood in the UK as long as they meet the standard donor criteria (haematocrit above minimum, well at time of donation, no recent infection). Giving blood every 12 to 16 weeks is an elegant solution for men with persistently high-normal haematocrit on TRT.

Some men find that routine blood donation every four months keeps haematocrit in range without any TRT dose change. This is a real win - free management of a side effect with a social benefit attached.

Register with NHS Blood and Transplant or, in Scotland, with SNBTS. Tell them you are on TRT if asked (it does not disqualify you). Donate when your haematocrit is above 0.50.

Venesection as therapy

If you are not eligible for voluntary blood donation (recent travel to areas with transfusion-transmissible infection, some medical histories, specific medications), therapeutic venesection is arranged through the clinic. The blood is discarded rather than used.

This is purely symptom-management, not a long-term strategy if dose reduction or form switching is available. We prefer to fix the cause (dose, form, apnoea) rather than rely on ongoing venesection.

Symptoms of high haematocrit

Most men with rising haematocrit are asymptomatic. Symptoms, if present, include: headache (particularly morning), facial plethora (reddening), dizziness on standing, blurred vision, poor concentration, fatigue, pruritus after warm showers. Any of these combined with a measured haematocrit over 0.52 warrants prompt intervention.

DVT and PE risk on TRT

Raised haematocrit increases venous thromboembolism risk. The TRAVERSE trial showed a small increase in pulmonary embolism risk on TRT vs placebo. The mechanism is likely multifactorial (erythrocytosis, mild hypercoagulability, blood pressure effects) with haematocrit as a major lever.

Patients at higher baseline VTE risk (personal history of DVT/PE, strong family history, known thrombophilia, recent hospitalisation or surgery, long-haul immobility) need careful baseline assessment and tighter monitoring.

Patients with active VTE should not start TRT. Patients with remote history of VTE can often start TRT after specialist assessment with careful haematocrit control from baseline.

When we stop TRT for haematocrit reasons

Rare. Most cases respond to dose adjustment, form change, or apnoea treatment. We stop TRT if:

- Haematocrit above 0.56 persistently despite all interventions - Symptomatic erythrocytosis not controllable by intervention - Patient preference for stopping rather than continuing with ongoing management

Alternatives to consider before stopping include switch to a lower-dose stable gel regimen, clomiphene as an alternative to exogenous testosterone, or adding HCG without full TRT.

The honest bottom line

Haematocrit rise is the most predictable side effect of TRT. It is manageable in almost all patients through dose adjustment, form smoothing, apnoea treatment, and occasional blood donation. Left unmonitored, it becomes a real cardiovascular risk. Monitored properly, it is a routine part of TRT care that rarely causes problems.

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