What can blood tests tell you, and what can’t they?
What can blood tests reveal about your health, and what are their limits? Learn what blood levels can show and why medical context matters when interpreting your results.
Blood tests can give you insight into your blood levels and certain processes in your body. A single blood sample can be used to measure several different values. But what can you actually learn from them?
The short answer: blood tests can provide clues about certain risks, deficiencies or imbalances. They cannot measure everything and do not replace a consultation with a doctor, a physical examination or imaging. What a result means depends on what was tested and your personal circumstances.
What do blood tests actually measure?
Your blood carries nutrients, waste products, hormones and immune cells, among other things. Examining these substances and cells gives an indirect picture of certain processes in your body.
A blood test result usually provides a clue about what is happening in your body. Creatinine, for example, is a waste product that your kidneys remove from your blood. The creatinine level can be used to calculate the eGFR: an estimate of how well your kidneys filter your blood.
What blood tests can tell you
Blood tests can provide information about your metabolism, organ function and possible deficiencies. Here are a few examples:
Iron stores and blood cell production. Haemoglobin helps establish whether you have anaemia. Ferritin provides information about your iron stores. A low ferritin level may indicate iron deficiency before haemoglobin falls.
Blood sugar over time. HbA1c provides information about your average blood sugar over the previous two to three months and complements a single glucose measurement.
Risk factors for cardiovascular disease. LDL cholesterol, HDL cholesterol and triglycerides provide insight into your fat metabolism. Elevated LDL cholesterol is an important modifiable risk factor for cardiovascular disease.
Kidneys and liver. Creatinine and eGFR provide information about kidney function. Liver enzymes such as ALT and GGT can provide clues about liver problems, for example those related to alcohol, medicines or fatty liver.
Thyroid function. TSH is often an initial test when investigating thyroid function. Its meaning depends on your circumstances and any additional tests.
Vitamin status. Vitamin D, vitamin B12 and folate tests can help investigate possible deficiencies. Whether an abnormal result explains symptoms requires medical assessment.
Inflammation. An elevated CRP level may be consistent with inflammation or infection. The result does not show where it is or what is causing it.
These are general examples. Not all the tests mentioned are included in every LiveLong package. The package overview shows which measurements are included.
Some abnormalities may be detectable before clear symptoms develop. This does not automatically mean that broad or regular testing improves health for everyone. Which measurements are useful depends on the question you want to answer and your personal circumstances.
What blood tests cannot tell you
The limitations of blood tests are just as important as their possibilities.
They do not directly show your heart and blood vessels. Cholesterol tells you about a risk factor, rather than how much atherosclerosis is present. Blood pressure, smoking, family history and lifestyle also matter when assessing your overall risk.
A single abnormal result is not automatically a diagnosis. Elevated CRP or a mildly abnormal liver enzyme can have different causes. A doctor considers your symptoms, medical history and other findings when interpreting the result and deciding whether further investigation is needed.
They do not measure your overall fitness or vitality. Your strength, how well you sleep and how you feel cannot be read directly from blood test results. Results within the reference range do not mean that every aspect of your health is good.
A routine preventive blood panel is not a general cancer screening test. Normal blood test results do not rule out cancer. Tumour markers are generally unsuitable for general screening in people without symptoms: they can cause false alarms and miss cancer. Targeted blood tests can play a role in cancer diagnosis or treatment. Organised screening programmes are available for certain types of cancer.
They cannot predict the future with certainty. Blood levels can provide information about risks, but cannot guarantee whether or not you will develop a disease.
Why blood levels can vary
Blood levels can change because of circumstances around the blood draw. Examples include food and drink, strenuous exercise, the time of day, hydration and a recent infection. For some hormone measurements, the menstrual cycle and stage of life also play a role.
Medicines and supplements can affect blood levels or test measurements. High doses of biotin, for example, can interfere with certain laboratory tests. Let the medical team know what you are taking.
An abnormal result needs to be assessed in the context of these circumstances. Sometimes a repeat test is appropriate; sometimes a different next step is needed. A doctor determines this based on the result and your circumstances.
What does a reference range mean?
Many results come with a reference range: the values found in most people in a particular reference group. This range often covers the middle 95 per cent of that group.
The more values you measure, the greater the chance that one will fall outside the reference range. This does not automatically mean you are ill. A doctor assesses the abnormal result alongside your other results and personal circumstances.
A value within the reference range also needs to be considered in context. Reference ranges can differ by laboratory, measurement method, age and sex. For some tests, a doctor also uses different decision thresholds to assess risk or treatment.
Why the same result can mean different things
Fictional example — not a real customer case
Two people have the same mildly elevated blood test result. One has recently recovered from an infection. The other takes medicines and has also had abnormal results in previous tests.
A doctor looks beyond the number, considering the other results, any symptoms and the medical history. The same value can therefore lead to a different decision about further investigation. This example shows why a result outside the reference range does not, on its own, support a conclusion about your health.
How can you get the most out of your blood test?
Follow the specific blood collection instructions from your doctor and laboratory. What is needed depends on the tests being requested.
Follow the fasting instructions if they apply to your test.
For hormone measurements, follow the instructions about the time of day and, where relevant, the timing within your menstrual cycle.
Report any medicines and supplements you take. Do not change these on your own initiative before the blood draw.
If needed, discuss whether recent strenuous exercise or alcohol use affects how you should prepare.
Have you recently been ill? Ask whether this affects the most suitable time for your blood draw.
Where possible, compare results obtained under similar circumstances.
Comparison with previous results can sometimes provide additional information. Whether repeat testing is useful depends on the value and your personal circumstances.
At LiveLong, a doctor assesses your blood test results alongside the information in your health questionnaire. You receive an explanation of the results and, where needed, a possible next step. Read more about the practical process in our article on blood tests without a GP referral and our frequently asked questions about the testing process and results.
What should you take away from your results?
Blood tests provide information about the values that were actually measured. Their value lies in careful interpretation, taking your symptoms, medical history and other relevant circumstances into account.
Do you have new or persistent symptoms? Contact your GP. Preventive blood testing does not replace that assessment.
Would you like more insight into your blood levels?
See which measurements our packages include and how a doctor assesses your results.
View the preventive blood test packages