After taking a blood sample, the blood needs to be managed in a proper way so as to prepare it for investigations.
Blood sample handling and processing can be summarized as below.
The first critical step in the lab testing process, after obtaining the sample, is the preparation of the blood sample.
Specimen integrity can be maintained by following some basic handling processes –
- The tubes should be filled up to desired level which is usually indicated on the tubes so as to ensure proper blood-to-additive ratio. In case of vacutainers, the tubes should be filled until the vacuum is exhausted and blood flow ceases. Blood should not be forced vigorously into tubes beyond/under the indicator mark.
- The blood filled tubes should be stored at 4-25°C (39-77°F).
- Expired tubes should not be used.
- Immediately after filling, the tubes should be inverted gently for a few times. Turn the filled tube upside down and return it to upright position. [see image below] This is one complete inversion.
- Proper mixing assists in the clotting process in case of gel and barrier tubes. In case of additive tubes, it ensures homogenous mixing of the additives and the blood.
A short summary of inverting the tubes is given below.
Number of inversions
[How many turns]
|Light blue top||Sodium citrate||3-4|
|Serum separator tubes (SST)||Gel||5|
|Green top||Sodium heparin||8-10|
|Grey top||Sodium fluoride||8-10|
|Red top||No additive||5|
- The tubes should not be shaken. Vigorous mixing may cause foaming or hemolysis making the sample useless. Insufficient mixing or delayed mixing in serum tubes may result in delayed clotting. In tubes with additives, inadequate mixing may result in platelet clumping, clotting and incorrect test results.
- After gentle inversion, the serum separator tubes (SST) should be placed vertically in a test tube rack for 30 minutes till the blood clots completely. Short clotting times can result in fibrin formation, which can interfere with complete gel barrier formation.
Centrifugation and Post-centrifugation Handling
Centrifuge at 4000 rpm for 10-15 minutes. Complete gel barrier formation (in case of gel barrier tubes) is dependent on time, temperature and gravitational force. The uniformity of the barrier is time dependent. Therefore reduced centrifugation time would result in an incomplete barrier.
Precautions while centrifugation
- Centrifuges must never be operated without a cover in place.
- Tubes should be well stoppered.
- Care should be taken to balance the tubes while placing them in centrifuge.
- Weight of buckets, tubes and their contents on opposite side of the rotor should not differ by more than 1%.
- Opposing tube holders must be empty or loaded with equally weighted samples (tubes of the same size and equal in fill).
- If an odd number of samples is to be spun, a tube filled with water should be used to match the weight of the unpaired sample and placed opposite to it.
- If an abnormal noise or vibration is noted while the centrifuge is in operation, the unit should be immediately stopped and checked for a possible load imbalance.
- For a horizonal, swing-bucket centrifuge, the recommended spin time is 10 minutes. For a fixed-angle centrifuge, the recommended spin time is 15 minutes.
- The speed control switch should be at zero before starting the centrifuge and adjusted to the required speed.
- The lid should not be opened while the centrifuge is in operation.
- Centrifuge should never be slowed down or stopped by grasping part(s) of the device with hands or by applying another object against the rotating equipment. It should be allowed to stop on its own.
- Centrifuge should be cleaned daily with a disinfectant and paper towel. Broken tubes or liquid spills must be cleaned immediately with 1% sodium hypochlorite solution.
- Gloves must always be worn while handling centrifuge and blood samples as any spillage can be hazardous.
The serum/ plasma should be physically separated from contact with cells as soon as possible and not later than 2 hours. If left unseparated, analytes shift between the cells and the plasma/serum resulting in erroneous results. Also glucose gets consumed by RBCs and so blood glucose estimation if performed later on an unseparated serum sample will be fallacious.