How to Read Your Blood Test Results: What UK Doctors Don't Always Explain

Why 'Normal' Doesn't Mean 'Optimal'

When your GP tells you your blood results are 'normal,' they mean your values fall within the laboratory reference range, a statistical spread that covers 95% of the population. But the reference range is not a health target. A ferritin level of 15 µg/L is technically 'normal' (reference range: 13–150 µg/L for women), but many haematologists consider anything below 30 µg/L as functionally deficient, and optimal levels are typically 50–100 µg/L. Similarly, a TSH of 4.5 mIU/L is within most lab ranges (0.4–4.5 mIU/L), but endocrinologists increasingly recognise that optimal thyroid function corresponds to a TSH of 0.5–2.5 mIU/L. Understanding this distinction is the first step to becoming an informed patient.

The Full Blood Count: What Each Marker Means

A Full Blood Count (FBC) is the most commonly ordered blood test in the UK. It measures haemoglobin (Hb), which should be 120–150 g/L for women and 130–170 g/L for men, values below these thresholds indicate anaemia, which affects approximately 1 in 4 women globally. White blood cells (WBC) range from 4.0–11.0 × 10⁹/L, with elevations suggesting infection or inflammation and very low counts requiring urgent investigation. Platelets (150–400 × 10⁹/L) affect clotting, both extremes matter. The mean corpuscular volume (MCV) tells you the size of your red blood cells: high MCV (over 100 fL) suggests B12 or folate deficiency; low MCV (under 80 fL) points to iron deficiency. This single test, properly interpreted, can flag dozens of conditions.

Thyroid Function: TSH, T4, and the Tests Your GP May Not Order

Standard NHS thyroid screening measures only TSH (Thyroid Stimulating Hormone). If TSH is abnormal, the lab may reflexively add Free T4. But for a complete picture, you need TSH, Free T4, Free T3, and thyroid antibodies (TPO and TG). Many patients with Hashimoto's thyroiditis have a 'normal' TSH but elevated antibodies, meaning autoimmune destruction is already underway years before the TSH becomes abnormal. If you have symptoms of thyroid dysfunction (fatigue, weight gain, cold intolerance, hair loss, brain fog) but your GP says your thyroid is 'fine' based on TSH alone, request the full panel. If they decline, private thyroid panels are available for £49–89 through providers like Medichecks or Thriva, with results in 2–5 days.

Vitamin D: The UK's Most Common Deficiency

The National Diet and Nutrition Survey found that approximately 1 in 5 UK adults has a vitamin D level below 25 nmol/L, the threshold for clinical deficiency. A further 40% have levels between 25–50 nmol/L, classified as 'insufficient.' The NHS reference range starts at 25 nmol/L, but the Endocrine Society and most international guidelines recommend maintaining levels above 75 nmol/L for optimal bone health and immune function. Current NHS guidance recommends 400 IU (10 µg) daily for adults, but this is a maintenance dose for people who are already replete. If you are deficient, you may need a loading dose of 20,000–50,000 IU weekly for 6–8 weeks (prescribed as colecalciferol), followed by a maintenance dose of 1,000–2,000 IU daily. Ask your GP for a 25-hydroxyvitamin D blood test, it costs the NHS approximately £5 and is one of the most cost-effective tests available.

Liver and Kidney Function: Early Warning Signs

Liver function tests (LFTs) include ALT, AST, ALP, GGT, bilirubin, and albumin. In the UK, non-alcoholic fatty liver disease (NAFLD) affects approximately 25–30% of adults and is the leading cause of abnormal LFTs. An ALT above 40 IU/L (the upper reference limit in most labs) warrants investigation, but recent guidance from NICE suggests that even levels above 30 IU/L in men and 19 IU/L in women may indicate early liver damage. For kidney function, the eGFR (estimated Glomerular Filtration Rate) is the key marker: above 90 mL/min is normal, 60–89 is mild impairment (often age-related), and below 60 requires monitoring and possible referral. Crucially, kidney damage is often silent until late stages, regular monitoring is especially important if you take NSAIDs regularly, have diabetes, or have high blood pressure.

How to Get Private Blood Tests in the UK

If your GP declines to order specific tests, or if you want a more comprehensive panel than the NHS typically provides, private blood testing is widely available. Medichecks and Thriva offer fingerprick home testing kits from £39 for basic panels to £199 for comprehensive wellness screens. For venous blood draws (more accurate for certain markers), companies like London Medical Laboratory, Blue Horizon, and private pathology departments at hospitals like The London Clinic offer walk-in services from £50–300 depending on the panel. When ordering private tests, choose UKAS-accredited laboratories to ensure quality. Take your results to your GP, they are obligated to review clinically significant findings, regardless of whether the NHS ordered the test.