Pre-analytical phase and tube requirements
The accuracy of laboratory diagnostics largely depends on the correct initial collection of biological material. For example, when performing a glucose tolerance test, exclusively venous plasma is used, and blood is collected into chilled vacuum tubes.
- The tubes are immediately placed on ice to maintain sample stability.
- Sodium fluoride (6 mg per 1 ml of blood) is used as a preservative, which blocks the enzyme enolase and stops spontaneous glycolysis.
- The anticoagulants used are EDTA or sodium citrate.
- Plasma separation by centrifugation must be performed no later than 30 minutes after blood collection, after which the plasma is transferred to a secondary tube.
Special tubes for hormones and peptides
The determination of specific biologically active substances requires special preparation and strict adherence to temperature and chemical conditions.
- Somatostatin, pancreatic polypeptide (PP), VIP, and glucagon: venous blood is collected into special vacuum tubes containing K2EDTA or K3EDTA combined with aprotinin.
- Serotonin and 5-HIAA: require strict avoidance of certain foods (bananas, tomatoes, coffee, chocolate) 2–3 days prior to the test.
- 24-hour urine for 5-hydroxyindoleacetic acid: must be collected with the mandatory addition of a preservative—citric acid.
Biochemical identification and monitoring
Classical and modern methods are used to determine the properties of microorganisms and metabolites:
- Hiss media help evaluate saccharolytic activity based on indicator color change and gas production.
- Proteolysis is studied by inoculation into gelatin stabs, coagulated serum, or milk agar.
- The carbohydrate breakdown pathway is determined using the O-F test with two tubes (one of them is covered with vaseline oil).
For serial studies and patient monitoring, it is critically important to use identical test systems within a single laboratory, as results obtained on equipment from different manufacturers cannot be compared.
Cold, ice, and fluoride: glucose will maintain its stability for 30 minutes in the tube.
Class and exam questions
Why is sodium fluoride added to the blood collection tube for glucose testing?
Sodium fluoride acts as an inhibitor of the enzyme enolase. This prevents spontaneous glycolysis and the breakdown of glucose in the collected blood sample.
Can the results of assays performed on test systems from different manufacturers be compared?
No, the results of different test systems cannot be compared. Serial studies for monitoring must be performed using identical test systems in the same laboratory.
What should be done with the plasma immediately after centrifugation?
After centrifugation, which is performed no later than 30 minutes after blood collection, the plasma must be transferred to a secondary tube.
What else to study on this topic
- Rules for patient preparation for chromogranin A and gastrin testing (discontinuation of proton pump inhibitors and histamine receptor blockers)
- Dietary guidelines for serotonin and 5-HIAA testing
- Methodology of the O-F test for determining glucose breakdown pathways
- Evaluation of saccharolytic activity using Hiss media
- Requirements for collecting 24-hour urine for 5-hydroxyindoleacetic acid
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