Note taking samples for biochemical tests

2021-07-05 04:24 PM

The collection of plasma for enzyme tests is necessary because while waiting for serum separation enzymes are abundant in red blood cells, platelets are easily released during coagulation, causing false results.

Defects in specimen collection techniques may result in incorrect test results. In order to have accurate and error-free test results, it is necessary to pay attention to some issues when taking samples as follows:

General requirements

Usually, blood is taken in the morning, after a night of waking up, have not eaten. Depending on the test requirements, it is necessary to prepare appropriate instruments and antifreeze so as not to cause erroneous test results. Each specimen should clearly state the patient's name and department to avoid patient confusion; examine the request.

Some specific requirements

Collect whole blood or plasma

Technical requirements need to draw blood so that it does not destroy blood, so it is necessary to pay attention to the following points: When pumping blood into the centrifuge tube, remove the needle, pump gently, and balance when centrifugation. Plasma should be separated within one hour after blood collection to avoid hypoglycemia, potassium can be released from red blood cells, causing hyperkalemia.

The collection of plasma for enzyme tests is necessary because while waiting for serum separation enzymes are abundant in red blood cells, platelets are easily released during coagulation, causing false results.

Take the serum

Take venipuncture, do not eat anything when fasting to avoid changes due to eating. When the blood is drawn, remove the needle, gently inject the blood into the test tube, place the blood in the 37°C incubators or at room temperature. Once the blood has clotted, use
a small, round-ended glass rod to gently separate the top of the clot from the wall of the tube so that the serum is separated more quickly. For some time for the serum to secrete, take out the centrifuge at 2500 - 3000 rpm, draw the serum into another test tube is best.

Use antifreeze

The amount of anticoagulant for 1 ml of blood is as follows:

Oxalate:  2 - 3 mg.

Citrate: 5 mg.

Flouride: 10 mg.

Heparin: 50 - 70 units.

EDTA:            1 mg.


Electrolyte tests do not use sodium oxalate, or citrate because it increases sodium content and reduces Ca++.

For fibrinogen testing, EDTA should be used for anticoagulation, not heparin.

The allowable storage time for serum or plasma is 4 hours at room temperature, 24 hours at 40 ° C.

For enzyme tests

After blood collection is completed, do the test as soon as possible, avoiding hemolysis (usually due to blood collection and centrifugation techniques). Long-term storage of blood increases the permeability of the red blood cell membrane. When preserving patient samples, attention should be paid to the time allowed to store serum or plasma at 40 ° C, according to the following table:

Table: Permissive storage times for serum enzymes determination.


Enzymes and specimens


The time allowed





Test early in the day


Early test


No more than 24 hours


Test the first hours


7 days


7 days


Test early in the day


Test early in the day


3-4 weeks


3 days


Within day


Within day


One weak


2 days


2 weeks


2 days


Take a test immediately






When collecting urine

Normally, midstream urine is taken, removing the first part to do qualitative tests, in which there are 10 parameters, 2 parameters and 3 urine parameters. When glucosuria is suspected, urine should be collected 2 hours after a meal.

24h (or 12h) urine for quantitative testing of some substances, usually must be collected in sterile instruments and preserved with a preservative such as 10% thymol solution (5ml) and combined in cold storage. Preserved thymol solution for most urinalysis (except 17-cetosteroid).

Specialized tests

Blood gas test and acid-base balance

To do blood gas test and acid-base balance, it is necessary to draw blood according to regulations and techniques to give accurate results. Some of the technical requirements are:

Blood collection site:

Angioplasty is best, usually from the ulnar, radial, or brachial arteries. It is also possible to draw capillary-arterialized blood from a heated heel, finger, or earlobe, resulting in almost the same results as arterial blood. Arterial capillary blood collection is especially good for children.


Collecting blood with a special instrument such as a micro-sample, it allows taking arterial blood to avoid air bubbles that affect the test results (pH, PaCO2, PaO2, SaO2...).

After taking blood, it must be measured within 30 minutes. For this, the instrument must be pre-calibrated and always ready for measurement. If due to conditions it is not possible to measure immediately, store blood samples in ice, temperature ≤ 4oC, and measure as soon as possible.