What do they take blood from a vein during a physical examination?

A general (clinical) blood test is usually one of the first tests prescribed for the purpose of making or specifying a diagnosis. Recently, in many laboratories take a complete blood count from a vein. In this regard, patients often have a question – why is blood taken from a vein, and not from a finger?

What do they take blood from a vein during a physical examination?

Indications for analysis

As a rule, blood is taken from the finger for a general blood test. But in some cases, a blood test from a vein is assigned for the study. This happens when the purpose of laboratory research is to identify a large number of parameters for which there is not enough blood from a finger. Also, more blood is needed to detect certain types of infections.

What do they take blood from a vein during a physical examination?

In addition, venous blood in its composition differs from capillary – the one that is taken from the finger. So, it contains more glucose, and this is important in many situations for a more accurate diagnosis.

Preparation for analysis

To prepare for the study of blood from a vein, it is enough to refrain from consuming heavy fatty foods and alcohol on the eve of the analysis. A blood test from a vein is taken on an empty stomach, usually in the morning. Do not eat at least two to three hours before visiting the lab. But it concerns the general blood test. Some tests have stricter rules. For example, before the biochemical analysis of blood can not eat 8 hours, and the time interval between the last meal and the time of blood sampling when determining the blood triglycerides must be at least 12 hours. Therefore, on the issue of preparation for the analysis you need to consult with your doctor.

Blood sampling is carried out using a needle from the veins of the forearm or elbow bend. If these veins are not clearly visible, blood can be taken from the venous vessels of the popliteal cavity or the dorsal surface of the hand.


Below are the main parameters of the general blood test, their standard notation, used in many laboratories, and the standards for blood analysis from a vein, as well as some explanations of deviations from the standards.

  1. Hemoglobin (Hb). The norm for men is 120–160 g / l, for women – 120–140 g / l. Low hemoglobin can be observed after bleeding, as a result of anemia and some hereditary diseases.
  2. Hematocrit (Ht). The rate for men is 40–45%, for women– 36–42%. This indicator indicates the percentage ratio of the number of blood cells (platelets, erythrocytes, leukocytes) to the volume of its liquid part – plasma. Low hematocrit occurs after blood loss, as well as in case of violation of the process of formation of new blood cells, for example, in autoimmune diseases and acute infectious processes. An increase in this indicator may indicate dehydration.
  3. Red blood cells. The norm for men is 4.3–6.2 x 10 12, for women – 3.8–5.5 x 10 12. Elevated red blood cell levels indicate a risk of red blood cells sticking together, which can lead to thrombosis (vascular blockage). Low red blood cell levels indicate a lack of oxygen.
  4. Color Indicator (CPU). The rate of this indicator is 0.85–1.05. Indicates the ratio of hemoglobin to the level of red blood cells. Deviations of the color index from the norm are detected in various types of anemia.
  5. Leukocytes (WBC). The norm is 4–9 x 10 9. This parameter of the general blood test may increase with infectious processes in the body and leukemia. A decrease in leukocytes can be a sign of impairment of the process of their formation in the bone marrow, which may indicate autoimmune, oncological and acute infectious diseases.
  6. Neutrophils (NEU). Norm – within 70% of the total number of leukocytes. A significant increase in the level of neutrophils usually speaks of the purulent inflammatory process in the body.
  7. Eosinophils (EOS). The normal content of eosinophils varies from 1–5% of the total number of leukocytes. Increased eosinophil level is characteristic in the presence of parasitic diseases, as well as in allergic diseases.
  8. Lymphocytes (LYM). Normal levels of lymphocytes in the blood – 19-30%. The increase in the number of lymphocytes occurs in infectious diseases and blood diseases. Low levels of lymphocytes may indicate renal failure, chronic diseases, reduced immunity, or medications that suppress the immune system.
  9. Platelets (PLT). The norm is 170–320 x 10 9. High platelet count can be observed after surgery and in some blood disorders. A decrease in platelet levels can be an indication of an acute inflammatory process or an immunological disease.
  10. Erythrocyte sedimentation rate (ESR). The rate of this blood parameter for men is 10 mm / h, for women – 15 mm / h. An increased erythrocyte sedimentation rate usually acts as an indirect sign of any abnormalities in the body, such as the inflammatory process.

What do they take blood from a vein during a physical examination?

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