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Blood test

Your doctor has prescribed laboratory tests by blood for you?

Here is some information to help you understand the steps involved in a blood test and prepare for it.

Regulations of the blood sample

In Quebec, blood samples are collected strictly by doctors, nurses and medical technologists. Respiratory therapists and radiology technologists are also authorized to perform certain blood tests in the practice of their specialty. The test must be prescribed by an authorized health care professional (doctor, medical resident, dentist, pharmacist, midwife or nurse). In Quebec, prescriptions from naturopaths, osteopaths, homeopaths, chiropractors and acupuncturists are not authorized. Prescriptions for analysis can be for an individual patient or a large group of individuals (collective prescription). Unless otherwise indicated by the physician, the validity period of a prescription does not expire.

Prescriptions from physicians practicing outside Canada are generally not accepted, except for specific reasons (e.g. a traveller requiring a blood test as part of regular monitoring of his medical condition). In the event of an extended stay, a new prescription from an authorized Quebec health care professional may be required.

Preparing for your blood test

Most laboratory blood tests do not require any specific preparation in advance. When necessary, the laboratory will inform you of basic precautions you should take before the test, but ultimately, it’s your attending physician who should notify you of any particular requirement. Some activities may affect results, such as recent or intense physical exercise, insufficient fluid intake (causing dehydration) or for some men, sexual activity (for the PSA test). It’s important to inform the staff at the blood test centre of any deviation from the specific requirements, so that this information appears in the final report and makes it easier to interpret (e.g. if you are not fasting or broke your fast before indicated, if you took certain drugs before the test, etc.). Also, make sure to bring your prescription signed by your health care professional!

Fasting

For an accurate interpretation of the results, fasting is only rarely required. Eating before a blood test can increase your levels of glucose and triglycerides, which are involved in calculating LDL cholesterol (bad cholesterol). However, automatic measurement of glycated hemoglobin when glucose levels are elevated, or calculation of the non-HDL cholesterol fraction, as now recommended by the Canadian Cardiovascular Society, can also be used to diagnose and monitor diabetes or cholesterol disorders in non-fasting individuals.

Procedure for taking blood

Once you have checked in using an official piece of ID with photo, you will be asked to make yourself comfortable in a partially reclined chair. We usually take blood from a vein in your arm. Once they have washed their hands, the assistant will briefly wrap your arm in a tourniquet (elastic band) to make your veins easier to see. You may also be asked to make a fist, so that the veins protrude more. The assistant will clean and disinfect the area before inserting a tube holder with a needle into the vein. The holder enables all the required tubes of blood to be filled with no need to change the needle. In certain cases where it’s too difficult to obtain blood from either arm, we may collect the sample through a fingertip using capillary blood sampling. This approach, which uses smaller collection tubes, is often indicated for very young children. Once every sample has been collected, the assistant removes the needle and applies gentle pressure to the area until bleeding stops. They may then apply a small adhesive bandage.

A label with your name and other relevant information is then affixed to each collection tube. It is crucial that you make sure each tube is identified with your name.

Side effects of the blood test

Blood tests usually do not cause any side effects. Some patients feel a slight sting or burning sensation when the needle is inserted into the vein of the forearm. Others might feel a little faint when the needle is removed. In very rare situations, especially if the procedure is difficult (e.g. the vein recedes or the needle moves), there might be swelling or bruising in the area where the needle was inserted. This will disappear within a few days.

Amount of blood collected

We collect only a tiny amount of blood. The lavender-capped tube used for the full blood count holds just 4 mL of blood, i.e. under a teaspoon. Even the largest tubes used for biochemical and other tests hold only about one teaspoonful. An expanded blood profile involving four or five collection tubes needs only one tablespoon (15 mL) of blood. This is a tiny fraction of the roughly four to five litres of blood in your body.

The tubes used in the test are identified by the colour of their cap. The colour indicates the presence of substances in the tube that either prevent coagulation (lavender, green or blue cap) or help the blood coagulate (yellow or red cap). Some tubes also contain a gel that helps separate the liquid part of the blood (serum or plasma) from the blood clot containing red blood cells, white blood cells and platelets).

The tubes are filled in a strict sequence, to prevent anticoagulants in one tube from contaminating the next. The staff at your Biron service centre are fully trained to follow the sequence: blue cap tube first, followed by yellow or red, then green, then lavender, etc. Each tube must immediately be turned upside down repeatedly, to blend the blood with the anticoagulant or clotting accelerator, depending on the test.

Stabilizing the sample

Depending on the tests prescribed and the distance the sample has to travel to get to the laboratory, the blood will need to be stabilized. Tubes containing anticoagulated blood (lavender or blue cap) are usually refrigerated until they are processed in the lab. Tubes of serum (yellow or red cap) must be centrifuged right away, to separate the serum before refrigeration. Best practices in sample stabilization stipulate that the temperature in the container used to transport the tubes be monitored, to protect the samples from excessive heat or cold.

A common practice at many sampling points is to refrigerate the samples BEFORE transport to a central laboratory, where the serum is then separated from the coagulated cells. This approach can lead to interference in certain tests, such as potassium, a vital electrolyte. Artefactually high potassium levels can raise concern and will often require that a new sample be taken. At Biron, our health care professionals make sure all conditions to stabilize your sample are satisfied before and during transport to our central laboratory.