This guide contains two important but different types of numbers: population reference ranges and guideline thresholds.
A population reference range describes where most people in a reference group sit. It is a description, not a target, and the reference group is not always a healthy one. Sometimes a range is simply taken from the manufacturer or the published literature.
A guideline threshold is different: it is a line a professional body has drawn deliberately, for a stated purpose, and we name the body and the purpose every time one appears.
Neither of the above can claim to be the optimal level at which humans should be. And they likely never will. Not least of all because everyone is so different and blanket statements or ranges almost never work.
For most tests there is no single NHS reference range. Each hospital lab publishes its own, which is why the same test can read differently in 2 different hospitals. Some of that difference comes from the analysers and methods each lab uses, and creatinine is the clearest example. The same applies to any blood testing labs but UK labs have agreed common ranges for a list of tests since 2011, including things like magnesium, albumin, ALP, adjusted calcium, creatine kinase and uric acid. Either way, the range on your own report is the one that matters.
All personal blood results should be discussed with your doctor.
How much does a repeat have to move before it means anything
Every result carries 2 kinds of wobble. The analyser is not perfectly repeatable, and your own level genuinely varies from one day to the next. Together they set a figure called the reference change value, which is how far a second result has to sit from the first before the difference is real and not noise.
It is larger than most people expect and it differs enormously between markers. For some it runs well past 50%. This is the reason a low testosterone is repeated before anyone acts on it, and why up to 30% of men whose first one comes back low are normal on the second. The same logic applies to every number in this guide, which is why a small change between 2 tests is not always an actual change in you.
Supplementing biotin can affect your blood results
Biotin is vitamin B7. Hair, skin and nail supplements routinely contain 5mg, at least 100 times the reference intake in the UK (50mcg) and 150 times in the US (30mcg), and prescribed biotin therapy can go well beyond that at 100 mg a day or more. This matters because a large group of blood tests are built on a biotin-based chemistry, and enough circulating biotin bends the result.
It does not affect everything, and where it does it does not push in the same direction. The tests affected are the immunoassays. On this panel that is testosterone, oestradiol, progesterone, SHBG, prolactin, FSH, LH, cortisol, DHEA-sulphate, anti-Mullerian hormone, TSH, free T4, free T3, total T4, thyroid antibodies, PSA, ferritin, vitamin D, vitamin B12, active B12 and folate. Which way it pushes depends on how the test is built. Tests for small molecules read falsely HIGH, so testosterone, oestradiol, progesterone, cortisol, DHEA-sulphate, free T4, free T3, vitamin D, B12 and folate all read higher than they are. The thyroid antibodies also read falsely high even though they are large molecules. This is because their tests are built the same way as the small-molecule ones. Tests for large molecules read falsely LOW, so SHBG, prolactin, FSH, LH, TSH, anti-Mullerian hormone, active B12, ferritin and PSA all read lower than they are.
That is why a biotin-affected panel does not look obviously wrong. It looks contradictory, and the contradiction is a pattern that ordinarily means something. High sex hormones alongside suppressed pituitary hormones is the classic one. A falsely low SHBG next to a falsely high testosterone makes it worse, because calculated free testosterone and the free androgen index are both worked out from those 2 numbers, so both get pushed the same way twice.
The rest of the panel is not affected at all. Liver, kidney, lipids, glucose, serum iron and the iron binding capacities, hs-CRP, the full blood count and HbA1c are measured by methods that do not use biotin.
The usual advice is to leave at least 8 hours after an ordinary supplement dose (5-10mg) and at least 3 days after the very high doses (100mg+) or the tests that are more sensitive to biotin like folate and thyroid antibodies. Manufacturers have been reformulating these tests to tolerate more biotin, and how much any one of them now tolerates depends on which version your lab runs, so the safe move is unchanged. If you take biotin and nobody asked you about it before the draw, stop it and repeat.
Fasting for Ramadan can affect your blood results
Ramadan is a lunar month in which observant Muslims take no food or drink, water included, from dawn to sunset, and in a UK summer that daily fast can run past 18 hours.
Most of the panel holds still across the month. In healthy adults, fasting insulin and the insulin resistance worked out from it are unchanged, fasting glucose falls by a trivial amount, and the average movement in the lipid panel is smaller than the analytical variation of the test itself. Weight falls by about 1 kg and is back within a few weeks of the month ending.
What does move is concentration rather than the thing being measured. A sample drawn late in a fasting day is more concentrated, because a long fast without water thickens the blood, so creatinine and urea read higher and eGFR reads lower, and haematocrit, haemoglobin, albumin and adjusted calcium shift the same way. Drinking more overnight brings them back, which is what shows this is concentration and not damage. Bilirubin goes the other way and rises with fasting in everyone, which matters most in Gilbert's syndrome and is covered in the Liver section.
In practice a test taken during Ramadan is usually done in the evening after eating, which is a fed sample. Anything that asks for a fast, so glucose and triglycerides, will not be a fasting value on that draw. A morning sample is drawn a few hours after suhoor and is not a fasting value either. If you want a clean fasting result, the week before the month or the week after it is the straightforward answer. This is behaviour and not ethnicity, so it applies to anybody who fasts this way.
This guide is reviewed regularly and updated to reflect current guidance and new evidence.
Last reviewed: 5 October 2026.
Every marker, grouped by system. Pick a section, or go straight to a marker.
Hormones
Testosterone | Free testosterone | Free androgen index | Oestradiol | Progesterone | Sex Hormone Binding Globulin | Follicle Stimulating Hormone | Luteinising Hormone | Prolactin | DHEA-Sulphate | Cortisol | Anti-Müllerian Hormone
Thyroid
Thyroid Stimulating Hormone | Free T4 | Free T3 | Thyroid Antibodies
Iron
Iron | Total Iron Binding Capacity | Unsaturated Iron Binding Capacity | Transferrin saturation | Ferritin
Full blood count
Haemoglobin | Haematocrit | Red blood cell count | MCV | MCH | MCHC | RDW | White blood cell count | Neutrophils | Lymphocytes | Monocytes | Eosinophils | Basophils | Platelet count | MPV
Liver
ALT | AST | ALP | GGT | Total bilirubin | Total protein | Albumin | Globulin | Lipase
Kidney
Urea | Creatinine | eGFR | Sodium | Potassium | Cystatin C | Uric acid
Lipids and cardiovascular
Total cholesterol | HDL cholesterol | LDL cholesterol | Non-HDL cholesterol | HDL as a percentage of total cholesterol | Triglycerides | Apolipoprotein B | Apolipoprotein A1 | Lipoprotein(a)
Blood sugar
Inflammation
High-Sensitivity CRP | Erythrocyte Sedimentation Rate
Vitamins and minerals
Vitamin D | Total Vitamin B12 | Active Vitamin B12 | Folate | Magnesium | Zinc | Adjusted Calcium
Other
Creatine Kinase | Prostate-Specific Antigen
References
Where a threshold, a reference range or a guideline position appears in this section, the body that published it is named alongside it in the text. Where the evidence and current guidance disagree, the entry says so instead of picking a side. The sources are listed here by marker.
Reference ranges and guideline thresholds
How much does a repeat have to move before it means anything
Harris EK, Yasaka T. On the calculation of a "reference change" for comparing two consecutive measurements. Clin Chem 1983;29(1):25-30
Fraser CG. Biological Variation: From Principles to Practice. Washington DC: AACC Press, 2001
Supplementing biotin can affect your blood results
Roche Diagnostics. Elecsys Ferritin method sheet, V7.0, 2023-12 (GB)
Ghosh A, Russell A, Dasgupta A. Newly reformulated total and free PSA immunoassay on cobas e411 analyzer is virtually free from biotin interference. Ann Clin Lab Sci 2022;52(3):504-506
Fasting for Ramadan can affect your blood results
This page is general information about what lab tests measure. It is not medical advice, it is not a diagnosis, and it is not a substitute for speaking to a clinician about your own results.
Dr Abir Awan PhD
Specialist Haematology Pharmacist
Doctorate in Molecular Pharmacology
Independent Prescriber