Laboratory tests (blood tests) performed by the general practitioner

What is it about ?

Over the years, technical examinations have become increasingly important in the diagnosis and monitoring of a large number of conditions. Blood and urine tests undoubtedly occupy an important position. Some tests are very cheap and easy to interpret; others are overpriced and very specialized. The question arises as to which tests can be carried out at the general practitioner and which ones belong to the field of specialists. This patient guide is mainly dedicated to the first group, and describes more particularly which tests are most indicated for a specific condition.

Which tests for which condition?

Cardiovascular risk

In people aged 40 to 75 years, cardiovascular risk is estimated based on existing risk factors, such as:

  • A: age (over 50);
  • B: blood pressure greater than 140/90 mmHg or taking drugs to regulate blood pressure;
  • C: smoking (from 1 cigarette per day);
  • D: type 2 diabetes;
  • E: a history of cerebrovascular accident (stroke) or heart attack (infarction);
  • F: a history of stroke or heart attack in a first-degree relative when he was still young (under 55 for a man, under 65 for a woman).

Then, a blood test is taken to measure the cholesterol level in all people over 50 and in people under 50 who have the following risk factors: B, F or familial hypercholesterolemia, an inherited form (genetics) of excess cholesterol in the blood.

Taking medicines to regulate cholesterol and / or blood pressure

Once cholesterol-lowering treatment (medicines that lower cholesterol levels, such as statins) is started, regular check-ups are done to assess the effects of the treatment. After three months, it is checked whether the target values ​​are achieved. If so, an annual check-up is sufficient to monitor cholesterol levels.

Statins can affect the liver and muscles. At the start of treatment, the function of the liver is therefore closely examined. Then blood tests are only needed if there are muscle complaints and / or possible liver damage.

When certain drugs are used to lower blood pressure (diuretics, angiotensin converting enzyme (ACE) inhibitors), kidney function and potassium levels should be monitored:

  • before starting treatment;
  • one to two weeks after initiation of treatment;
  • each time the dose is adjusted.
Arterial hypertension

Blood sugar, kidney function and cholesterol levels are assessed for possible organ damage. In addition, a urinalysis is performed for protein and blood.

For the monitoring of high blood pressure without complications or organ damage, an annual blood test is sufficient to monitor cholesterol and kidney function.

Pulmonary embolism, infarction and heart failure

If you suspect pulmonary embolism (presence of blood clots in the blood vessels of the lungs) and coronary heart disease (heart attack or infarction), you will be referred directly to the hospital, where various blood tests will be performed.

However, if there is a low risk of pulmonary embolism, a blood test (measuring the concentration of D-dimers) may be performed to rule out the disease. Is the amount of D-dimer in the blood normal? There is no question of pulmonary embolism. If the concentration is high, additional examinations are necessary.

If heart failure is suspected, you will be referred to the heart specialist (cardiologist). Again, a blood test (NT-pro BNP or BNP) can be done to rule out the disease.

Diabetes

Early detection of early stage diabetes is essential. This is why certain population groups are systematically screened. This screening involves a measurement of the level of sugar in the blood (glycemia) on an empty stomach.

The following people are screened for diabetes annually:

  • all seniors over 65;
  • women with a history of gestational diabetes;
  • people with a history of hyperglycaemia from stress, for example following a serious accident, surgery or hospitalization;
  • people with disturbed fasting blood sugar (100-126 mg / dl).

In addition to this, screening also extends to people aged 18 to 45 who meet two of the following conditions as well as to people aged 45 to 65 who meet only one of these conditions:

  • have delivered a baby weighing 4.5 kg or more;
  • have a first-degree relative (parent, child) with diabetes;
  • have a BMI ≥ 25;
  • have a waist circumference> 88 cm (in women) or> 102 cm (in men);
  • be on long-term treatment with cortisone or drugs that lower blood pressure.

If the blood sugar is normal (<100 mg / dl), a check-up every 3 years is sufficient.

Another screening method consists of measuring the level of HbA1c, which is considered abnormal as soon as it is greater than or equal to 6.5%. The HbA1c level gives a good representation of the average blood sugar level and is used in the monitoring of people with diabetes. However, this blood test is not (yet) reimbursed for non-diabetics.

In the monitoring of people with diabetes, the blood sugar level (fasting blood sugar and HbA1c) is checked every 3 months. In addition, the following laboratory tests are carried out once a year:

  • cholesterol and other fats in the blood (triglycerides);
  • kidney function;
  • presence of albumin (= small protein) in the morning urine.
Obesity

Being overweight especially increases the risk of cardiovascular disease and diabetes. The basic tests therefore concern the control of fat in the blood (cholesterol and triglycerides) and blood sugar. These values ​​are measured in all people with a BMI greater than 30.

Liver disease

Liver function tests are done if you suspect or show signs of liver disease, as well as in people who have risk factors such as:

  • risky sexual behavior (unprotected sex with several partners);
  • intravenous drug use (injected into the veins);
  • a blood transfusion received before 1992;
  • an accidental needle stick;
  • long-term, scheduled use of drugs that lower cholesterol (statins) or drugs that may be toxic to the liver.

In addition to liver function, antibodies against hepatitis A, B and C and against HIV can also be determined.

Pregnancy

Pregnancy is detected by means of a urine test. The blood test is only necessary if in doubt.

At the start of pregnancy, a fairly extensive blood and urine test is performed. Indeed, it is important to detect the conditions which can influence the development of the baby. The following laboratory tests are performed:

  • blood group, irregular antibodies and rhesus factor;
  • red blood cells, hemoglobin and hematocrit;
  • the iron (ferritin) level if the hemoglobin level is less than 9.5 g / dl;
  • antibodies against rubella, toxoplasmosis, hepatitis B, syphilis and HIV;
  • bacteria in the urine (around the 12 th or 16 th week of pregnancy);
  • proteins in the urine (at each check-up).

Detection of antibodies against CMV (cytomegalovirus) is not recommended.

Some of these tests are repeated during pregnancy.

Around the 24 th  week of pregnancy, a pregnancy diabetes screening is performed using a glucose solution, you need to drink, and a measure of blood glucose an hour later. It is not necessary to be fasting for this test.

Prenatal testing for birth defects via the DPNI (prenatal screening noninvasive) is conducted with the permission of parents and, through a blood test around the 12 th  week of pregnancy.

Unexplained fatigue

If fatigue persists for more than a month and cannot be explained by any acceptable reason or if it concerns a person over 65 years of age, a blood test will be performed to assess the level of hemoglobin, blood sugar levels. fasting and the function of the thyroid gland. The iron (ferritin) level will be checked in women of childbearing age.

Chronic renal failure

Kidney failure (decreased kidney function) is a condition that often goes hand in hand with and / or is often the result of other conditions. This is why it is, in principle, always detected in all people suffering from diabetes, high blood pressure and / or cardiovascular diseases, as well as in all people with a family history of these conditions.

The kidneys filter the blood by removing waste and excess water. The basic test is therefore to measure the amount of urine that the kidneys can filter per minute (eGFR or estimated glomerular filtration rate). This amount is calculated using a formula that takes into account the level of creatinine in the blood and in the urine. Creatinine is a waste that comes from the breakdown of muscle tissue. It is a question of decreased kidney function when the amount of purified urine per minute is less than 60 ml. We speak of renal failure as soon as this quantity falls below the 15 ml mark.

In addition, we also measure the amount of protein lost in the urine. This test is done every year in all people at risk. If the test gives a questionable result, it is repeated twice. The diagnosis of chronic renal failure is only made after 3 abnormal results over a period of 90 days. At a later stage of the disease, a more thorough blood test measures, among others, hemoglobin, calcium, phosphorus and vitamin D.

Sexually transmitted infections (STIs)

For screening for STIs such as chlamydia, gonorrhea, herpes and trichomonas, a cotton swab is used to collect some fluid or phlegm from the ureter, cervix, wall of the vagina, or the lesion itself. .

In men without symptoms, screening for Chlamydia infection and / or gonorrhea can also be done with a first-jet urine test (after 2 hours without urinating). This test is less reliable in women.

The different types of hepatitis (B and C), syphilis and HIV can be detected by looking for antibodies in the blood.

Thyroid Diseases

Thyroid function tests are only recommended if there is a real suspicion of thyroid disease and in the event of atrial fibrillation (heart rhythm disorder). The main test is to measure TSH (or thyroid stimulating hormone), a hormone that regulates the activity of the thyroid. If the TSH level is abnormal, the doctor will also measure the thyroid hormone itself, which is free T4.

When initiating therapy for thyroid disease and adjusting the dose of the drug, a new TSH and T4 measurement should be scheduled (at the earliest) after 6 weeks. The reason for this delay is that it takes some time for blood levels to stabilize. When the dose is well established, control tests are done every three months for a year. Afterwards, an annual check is sufficient.

Taking conventional anticoagulants (warfarin)

The evaluation of the clotting time makes it possible to measure the thickness of the blood. This test is repeated every 3 to 4 days until the dose has stabilized. Then the checks are carried out every month. In some patients, the effect of anticoagulants can be very variable. Eating a diet rich in potassium and taking certain medications can influence the clotting time. These people are likely to need more regular check-ups.

Anemia

Blood is made in the bone marrow. Anemia can be caused by loss of blood, for example during bleeding, or by a limited production of new cells that make up blood in the bone marrow. In the second case, there can be two causes: a lack of components necessary for the production of red blood cells, such as iron, folic acid and vitamin B12, or a bone marrow condition such as leukemia . The main test used to diagnose anemia is measuring hemoglobin (Hb), the substance that gives blood its red color.

If the hemoglobin level is too low, additional tests should be performed to identify the exact cause: measurements of white blood cells, platelets, iron levels, folic acid and the amount of vitamin B12 as well. that liver and thyroid function tests may be needed.

Drop

Gout is caused by the presence of uric acid crystals in the joint. The measurement of the level of uric acid in the blood is unreliable. In an acute attack of gout, the test gives a normal result in half of the cases. The best test is to look for the presence of uric acid crystals in the fluid in the joint (synovial fluid), taken through a puncture.

Rheumatoid arthritis

Laboratory tests performed by the general practitioner cannot confirm or rule out rheumatoid arthritis with certainty. If the doctor has a strong suspicion on the basis of your symptoms and the observations made during a clinical examination, this is sufficient reason to refer you to the rheumatologist.

Diarrhea

In the presence of acute diarrhea lasting less than 10 days, no test is normally necessary. Most often these are infections of viral origin, which will go away on their own. On the other hand, a stool analysis is performed in the following cases:

  • diarrhea containing blood and accompanied by high fevers and a strong feeling of ill-being;
  • weakened immune system (eg in people with AIDS);
  • diarrhea containing blood in a child under 6 months of age;
  • suspected food poisoning in collective establishments;
  • recent stay abroad ((sub) tropical and Mediterranean regions).

In the case of chronic diarrhea that lasts more than 10 days or that recurs constantly, a stool test is done to look for the presence of parasites. At the same time, blood tests are also carried out to detect a possible gluten intolerance (celiac disease).

In older people with diarrhea, a blood test may be necessary to assess their state of dehydration.

How is a blood test done correctly?

The blood collected is divided into several tubes, each one intended for a different test.

Some tests should be done on an empty stomach, others not. The doctor will tell you in advance. Whenever possible, it is advisable to schedule a blood test in the morning, on an empty stomach, because all tests are then possible.

Blood samples are stored at room temperature. Usually they keep for at least 24 hours, but in principle they are always harvested within 6 hours. A small number of tests require the blood to arrive at the laboratory within a specified time. In this case, the doctor should make a specific appointment to organize the collection of the fresh sample.

Sources

Belgian practice guide ‘Request for laboratory examinations by General Practitioners
‘ (2011), updated 01.01.2011 – SSMG – Société Scientifique de Médecine Générale