Blood tests evoke a lot of wrong ideas. Take the ‘full blood examination’, for instance: ‘When patients talk about it, they often think: I want to get a lot of laboratory findings to confirm that I am well’, says Dr Uwe Popert, section spokesperson for general practitioners’ surgeries at the German College of General Practitioners and Family Physicians (DEGAM). ‘If I were to determine everything possible from your blood, you would be both poor and dead.’ Because nobody has enough blood in his or her body for all the tests possible. And they would cost an enormous amount of money.

The so-called full blood examination by itself costs about 100 Euro. And the term is misleading. The laboratory results by no means give us an all-encompassing idea of our blood. Or our general health status. What it does show, however, is the content of individual blood cells. In difference to the full blood examination without differential, the different types of immune cells are also identified. The expert jargon calls it a ‘differential blood count’ detailing about 14 blood values.

Almost half of our blood is made up of cells. But it contains a lot more. A lot of things can be tested for. ‘We can analyse about 2,500 blood components and other substances’, says Professor Jürgen Durner, who runs a large laboratory in Munich. The other substances include, for example, heavy metal manganese or pathogens like hepatitis viruses. For many laboratory findings, there are established normal ranges. Diversions might point towards certain diseases.

‘Us doctors reasonably limit ourselves to blood tests making sense in the specific situation’, Popert explains. If a patient, for example, is noticeably tired and also quite pale, it could be a sign for anaemia. Anaemia can be diagnosed by identifying low levels of haemoglobin in the blood, an oxygen-binding protein. The most common cause of anaemia is iron deficiency. Further test can confirm if that is the case.

Doctors use blood tests on many occasions to confirm their suspicion of a certain disease – or to refute it. The general health check to enable an early diagnosis of diseases also includes blood analyses.

However, blood tests are quite often arranged without good and concrete reason. The German College of General Practitioners and Family Physicians is critical of an overuse of certain tests. Popert names vitamin D checks, first and foremost. ‘Studies indicate that there is no advantage in propping up your blood levels with vitamin D medication as a precaution’, the general practitioner argues. Therefore, it is also useless to check whether vitamin D levels are below the normal range. Only in case of atrophy of the bones, we have clear indications that vitamin D medication is beneficial.

And there are another examples of regularly conducted blood test with questionable reason: the antibody test for borrelia, the cause of Lyme disease communicated from ticks to humans, for instance. ‘That only helps us if the patient has typical symptoms – like arrhythmia or inflamed joints’, Popert says. The antibodies by themselves have no meaning. They can be identified in the blood for decades – and often after an infection the patient had not even noticed.

At the moment, many people get tested for antibodies to the Corona virus. In our interview, Professor Christine Falk, President of the German Society for Immunology, explains why the test gives us little information on the body’s defences against a Covid-19 illness. In the following sections, you can find out what certain blood tests tell us about the condition of our inner organs and our immune system.

Heart and Vessels

‘We cast an especially careful eye over two blood values’, says Professor Christiane Tiefenbacher, head physician for cardiology, angiology and pneumology at the Marien Hospital Wesel near Dusseldorf. The tests help to identify or preclude a heart attack or a cardiac insufficiency. In case of a heart attack, cells die and release the muscular protein troponin. In case of a cardiac insufficiency, the stressed-out heart releases higher amounts of so-called brain natriuretic peptide (BNP) into the blood. ‘The substance causes the kidneys to discard increased amounts of natrium’, Tiefenbacher, also a leading member of the German Cardiac Society, explains. BNP lowers the blood pressure and widens the blood vessels. The heart attempts to support itself by releasing the substance.

Troponin and BNP help hospital doctors to diagnose heart problems. There are, however, far more blood values acting as important signifiers for the condition of our heart and our blood vessels. Take blood fats, for instance. They get evaluated as part of the general health check-up for everybody aged 35 or over: total cholesterol (TC), LDL cholesterol (LDL-C), HDL cholesterol (HDL-C) and triglycerides. High values for total cholesterol, LDL cholesterol, and triglycerides increase the risk of heart and vessel problems.

‘If somebody’s triglycerides are too high, he or she has an unhealthy diet, as a rule, does not exercise enough and/or drinks too much alcohol’, Tiefenbacher says. Doctors can aid their patients if they are willing to live more healthily. The blood level of LDL cholesterol is mostly genetically prescribed. A healthier lifestyle has only a limited impact upon it. However, LDL cholesterol can be lowered with the help of medication. So-called statins are the most common active agents of choice. Whether a doctor recommends cholesterol-lowering medication depends on the individual risk of heart or vessel diseases.

As part of the general check-up, the fasting value of blood sugar is also established. Patients should forgo food and drink for eight to twelve hours before the blood extraction. Otherwise, the blood sugar value is of little explanatory power. Values of more than 100 milligrams of glucose per decilitre of blood are an indication for an existing diabetes or its preliminary stage.

In case of suspected diabetes, the doctor will – as a rule – also determine the glycated haemoglobin value, called HbA1c in short. It indicates the average sugar blood count over the past few months. The reason: some of the sugar connects with the metalloprotein haemoglobin (Hb) in the red blood cells. These circulate through the vessels for about 120 days and are then discarded by the spleen. Doctors also measure the HbA1c value to determine whether a diabetes therapy is successful. Well-controlled blood sugar is vital for the well-being of heart and vessels.

Another blood value is relevant for heart patients taking the anticoagulant drug phenprocoumon. As part of their therapy, they have to get their coagulation value checked regularly. For the laboratory testing, a sample of their non-coagulating blood is clotted. Then it is checked for its activated partial thromboplastin time (aPTT). The result depends on the specific conditions of the test, however. The value gets more comparable and thus more diagnostically conclusive when applied to the common pathway of coagulation, resulting in the so-called international normalised ratio, or INR. The dosage of the anticoagulant drug phenprocoumon is then adjusted according to the test results. It has to be adjusted to prescribed INR values: In a therapy against auricular fibrillation, for example, the INR should be between 2 and 3, as a rule.


If the metabolic organ gets harmed, its cells emit certain proteins into the blood at a higher level, so-called hepatic (or liver) enzymes. If we talk about a liver function reading, it is usually for the following three enzymes: alanine aminotransferase (ALT or ALAT), aspartate aminotransferase (AST or ASAT) and gamma-glutamyl transferase (GGT or g-GT).

Doctors determine these values if they suspect diseases like fatty liver and hepatitis or problems with the gall bladder or the biliary tract. Such tests get ordered if common risk factors for a fatty liver exist – like type 2 diabetes, overweight, a sugary diet or heavy drinking.

‘Quite often, the doctor notices the fatty liver on a ultrasound scan and orders a liver function reading’, Professor Andreas Geier, head of hepatology at the University Hospital Würzburg, explains. Enhanced values by themselves, however, say very little about the condition of the liver. For a diagnosis, the physician usually does other check-ups. In case of an active liver disease, these check-ups tend to get repeated over time.

That includes controlling the enzyme values. They also get routinely checked if a patient is on medication harmful to the liver. These include certain antibiotics, antidepressants, cholesterol-lowering drugs and the painkiller paracetamol.

If your doctor advises lifestyle changes, it can also make sense to repeat a liver function reading after a while to see if the changes take effect. The levels tend to normalise if you reduce your alcohol intake drastically and forgo sugary soft drinks. Expert Geier opines: ‘If the patients experience that their liver values improve, it can work wonders for their motivation to change things in the long-term.’


Our kidneys filter about 1,800 litres of blood per day to make sure the metabolites of nutrients, muscles and genetic substance do not harm the body. In order to ascertain how well the kidneys are working, the blood is tested for such metabolites.

One of them is creatinine. It stems from the metabolisation of the energy supplier creatine in the muscles. The creatinine concentration in the blood serum can be different from person to person, depending on body size, musculature and diet. Individual outliers thus do not necessarily indicate a renal insufficiency. Serum creatinine, however, does allow for an approximation of the so-called estimated glomerular filtration rate (eGFR). It is also a good indicator in monitoring a diagnosed renal insufficiency.

One outlier in the blood values does not necessarily indicate an insufficiency of the filtration organ.

The eGFR is a more commonly used indicator than creatinine for the functioning of the kidneys. Permanently low values indicate a severe insufficiency. In most cases, blood purification – or renal dialysis – becomes necessary to treat that insufficiency, then.

Other values indicating the functioning of the liver are the concentration of urea and uric acid. Urea is a by-product of protein breakdown in the body. It indicates the level of filtration of all metabolic products. Uric acid is a by-product of the metabolisation of genetic substances and can cause kidney stones and gout. About 15 per cent of all kidney stones are caused by uric acid.

Finally, it is worth one’s while to look at the potassium level in the blood serum. If kidney function is in decline, the potassium level could rise, leading to cardiac arrhythmias.

Thyroid Gland

The hormones of the thyroid gland, impacting on growth, development and metabolism, play a central role in our well-being. To find out how well the organ just below our larynx is doing, doctors first look at one indicator: the thyroid-stimulating hormone (TSH).

The hormone is not produced by the thyroid gland itself, but by thyrotrope cells in the anterior pituitary gland, a major organ of the endocrine system just below the brain. TSH stimulates the production of the real thyroid hormone, triiodothyronine (T3). Initially, a T3 antecedent is produced, tetraiodothyronine, containing an extra iodine atom and thus shortened to T4. The thyroid gland converts a fifth of all T4 into T3. The remaining 80 per cent of T4 migrate into the surrounding tissue and only then get converted into T3.

T3 and T4 initially latch onto transport proteins in the blood. However, only the uncombined hormone is effective in the body. This free T3, shortened to fT3, only makes up 0.3 per cent of all T3 in the body. The share of free T4 (fT4) is even smaller. Measuring such small amounts is always sensitive to errors; changes are difficult to prove.

Even a small change in the level of free thyroid hormones leads to a major change in the TSH level.

However, as even minor changes in free T3 levels lead to major changes in the TSH levels, a higher or lower TSH level in the blood can give doctors an indication of a thyroid hyperfunction or thyroid deficiency – long before the actual symptoms come to the fore.

If the TSH level is well outside the parameter, it makes sense to determine the actual thyroid hormones. However, once again the reference values for TSH strongly vary depending on age, sex, ethnicity, pregnancy and iodine provision. People aged over 60 often have a rather high TSH level without suffering from a thyroid deficiency. The level of free thyroid hormones would then be within the reference levels.

In order to ascertain whether the thyroid function is abnormal due to a malfunction and what the causes are, you do need further examinations.

Immune System

In contrast to the heart, kidney or liver, our immune system is a hard-to-pin-down organ. It spreads all over our body. Nevertheless, blood can tell us a thing or two about the body’s own defence. For instance whether the body is momentarily fighting an infection.

Initially, the leucocytes (or white blood cells) get counted. These immunocompetent cells are produced in the bone marrow and constitute the cellular defence of the body. If the total number of leucocytes is reduced, it can be an indication of a higher consumption of leucocytes, as it happens with some viral infections like rubella or parasitic diseases like malaria.

If the number of leucocytes is higher than normal, there can be a number of reasons. The next step would be a full blood examination, or differential blood count. For that, six cellular types get accounted for individually. The three most important cellular types are the granulocytes. Under the microscope, they show a granular structure. Up to 80 per cent of all leucocytes are made up of the most common form, neutrophil granulocytes. If their number increases exponentially, that might be an indication of a bacterial infection or a toxicopathy.

Inflammatory processes often go hand in hand with a higher blood sedimentation reaction (BSR), often shortened to blood sedimentation. To measure the BSR, sodium citrate is added to a blood sample as an anticoagulant. The laboratory then measures how far the red blood cells sink withing an hour. In case of an inflammation, the sedimentation reaction is accelerated, because the proteins of the immune system clump together with the red blood cells.

However, an accelerated blood sedimentation does not necessarily indicate a health problem. In contrast, a strongly enhanced serum level of C-reactive protein (CRP) almost always indicates an illness. Its concentration grows in case of infections, heart attacks or tumour diseases. The CRP level is particularly helpful in follow-up check-ups: If the level decreases, the therapy is effective.

Proteins, differential blood counts and blood sedimentation reactions often provide doctors with an important indication of a general inflammatory response, but they do not help in finding the underlying cause. For a concrete diagnosis, further examinations are imperative.

Interview with Professor Christiane Falk, Hanover Medical School, President of the German Society for Immunology.

‘For the vast majority, tests for Corona antibodies are superfluous.’

Laboratory tests are supposed to measure the level of immunity against Covid. They rarely make sense.

Professor Falk, how useful are tests for antibodies against Corona viruses (SARS-CoV-2)?

If you want to know if somebody has been affected, the blood is checked for antibodies directed against a protein component of the pathogen not included in the vaccine. That is a protein from the shell of the virus. You would only find antibodies against this particular protein with recovered persons, not with every vaccinated one. If you are vaccinated, you only build antibodies against another protein of the virus, the spike protein. That is the part of the virus most vaccines are directed against. If you are vaccinated but not recovered, your blood only contains antibodies against this spike protein.

If a vaccinated person has only very few antibodies in their blood – what does that tell us?

Very little, actually. If people with a sound immune system get vaccinated, they build antibodies after every vaccination dose. However, the number of these antibodies in the blood decreases with time. Also, the strength of the antibody response is quite different from person to person. With some, it is quite strong, with others less so. Nevertheless, most people are protected.

And why is the antibody response so individualised?

Everybody has a kind of library of antibody factories in the body – the B cells. This library gets built according to the random principle, and is thus different for everybody. If somebody gets vaccinated, B cells with the appropriate antibodies latch onto the spike protein and get activated. They produce their antibody type en masse, then. The amount of these antibodies in the blood increases, depending on how many suitable B cells there are and how well they got activated. The activated cells develop into memory cells. With their next contact with the spike protein, and thus contact with the virus, they get reactivated quickly. But in-between, the number of antibodies in the blood declines.

So, is an antibody test necessary, then?

In the majority of cases, the answer is obvious: no! Most people are in good immunological health. That means that they react well to the vaccination. They might get infected and have Covid with a mild course – but their body’s defence controls the infection. For this vast majority of people, an antibody test is superfluous.

In many studies, antibody readings are part of the research. Why?

In relation to the so-called immune evasion of the virus, specially with the omicron variant, antibodies were in the limelight. ‘Immune evasion’ is the term for the ability of a pathogen to change and thus to avoid the antibody response in part. It is all about neutralising antibodies. They latch onto the virus so well that the virus cannot infect any more cells. In contrast to the original virus we got vaccinated against, we do not have as many neutralising antibodies against the modified omicron variant. That was proven in the studies. But neutralising antibodies are not the same as the general immunity of a human being after three vaccination rounds.

So what else is part of the general immunity?

We have got other antibodies that are absolutely important. They, too, are directed against the spike protein of the virus, but against other parts thereof. If you imagine the spike protein as a key to unlock cells, these antibodies might be directed against the key handle. They do not neutralise the virus. But they obstruct it by mobilising white blood cells that attack the virus and eliminate it.

What happens if the virus still infects the cells?

Even then, we are protected against severe illness. Our body can identify infected cells and eliminate them specifically. For that task, we have another type of immune cells, the T cells. They are the second line of defence behind the virus-intercepting antibodies. T cells make sure that the virus does not spread throughout our body. And that it does not cause severe illness. That has always been the intention of the vaccinations: avoiding severe illness. And T cells also generate a memory. If we get infected with the virus despite three vaccination rounds, our immunological memory prevents us from a stint in hospital.

So, should we measure the T cell response instead of the antibodies, then?

A test for T cells against the virus is quite elaborate. We cannot offer such an examination in every general practitioner’s surgery or every pharmacy. They get done in specialised laboratories. And in research – proving quite clearly that vaccinations generate a good T cell immunity.


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