When a blood sample is collected for laboratory analysis, the time between collection and testing is known as the pre-analytical phase. This period is highly influential in determining the reliability of the final result. Any delay or improper handling during this window can compromise specimen integrity, making the measurement inaccurate. Maintaining the sample’s original condition is paramount because the blood tube is essentially a temporary environment. The ultimate goal is to ensure the biochemical composition measured accurately reflects the patient’s status at the moment of collection. Understanding how long blood can remain viable in a tube is fundamental to quality patient care.
How Collection Tubes Stabilize Blood
Specialized collection tubes are designed with various chemical additives to temporarily preserve the sample or prepare it for separation. Tubes containing anticoagulants prevent the natural clotting cascade, keeping the blood in its liquid state. For instance, ethylenediaminetetraacetic acid (EDTA) works by binding calcium ions, which are necessary cofactors for many steps in the clotting process.
Similarly, heparin salts act by activating a protein called antithrombin, which then inactivates key clotting factors like thrombin and Factor Xa, halting coagulation. These anticoagulants allow for whole blood analysis or the later separation of plasma, the liquid component that still contains clotting factors.
Other tubes are designed to promote clotting to yield serum, the liquid portion of blood that remains after the cells and clotting factors have been removed. Tubes with clot activators, such as silica particles, accelerate this natural process. Serum separator tubes (SSTs) also contain an inert gel barrier that separates the heavier blood cells from the lighter serum after centrifugation, aiding in initial preservation.
Why Sample Integrity Declines Over Time
Despite the stabilizing efforts of tube additives, biological processes within the collected blood continue, leading to time-dependent degradation. One significant issue is hemolysis, which involves the rupture of red blood cells. When red cells break down, they release their intracellular contents, such as potassium and the enzyme lactate dehydrogenase (LDH), into the surrounding plasma or serum.
This release artificially elevates the measured concentration of these substances, leading to falsely high results for routine chemistry panels. Hemolysis can also interfere with the spectrophotometric reading used by laboratory instruments, making certain tests impossible to perform accurately.
Another major concern is glycolysis, where blood cells continue to metabolize the glucose present in the sample. If a sample sits for too long without being processed, this cellular consumption causes the measured glucose level to drop steadily over time. A delay in processing can therefore lead to a falsely low reading, which is problematic for diabetes diagnosis and management.
Over time, the morphology and viability of cellular components also change, impacting tests like the Complete Blood Count (CBC). White blood cells and platelets begin to degrade, which can affect accurate cell counting and differentiation if the analysis is significantly delayed. Furthermore, the activity of certain enzymes, such as alkaline phosphatase, can either increase or decrease in the sample over time, complicating the accuracy of delayed chemistry results.
General Time Limits for Common Tests
The maximum allowable time a sample can sit before testing varies significantly based on the specific test and the sensitivity of the analyte to degradation.
Hematology (CBC)
For general hematology tests, such as the Complete Blood Count (CBC) collected in EDTA tubes, the sample is typically stable for up to 24 hours when stored at room temperature. However, certain parameters, like platelet counts and blood cell morphology, may begin to show changes sooner, sometimes within 6 to 8 hours, requiring earlier analysis if a peripheral blood smear is necessary.
Routine Chemistry
Routine chemistry panels, which use serum or plasma, are generally stable for approximately 8 hours at room temperature if the cells and liquid phase remain unseparated. This limit is imposed because cellular activity continues to influence the plasma composition, causing shifts in analytes like phosphorus and ammonia. Once the sample is centrifuged to separate the serum or plasma from the blood cells, the stability window extends considerably, often allowing for refrigeration up to 48 hours or longer for many common analytes, including liver and kidney function markers.
Coagulation Studies
Coagulation studies, which measure the function of delicate clotting factors in a citrate tube, have much stricter time constraints. Because factors like Factor V and Factor VIII degrade quickly, a sample must usually be tested or processed within 4 hours of collection if it remains unspun. If testing is delayed beyond this narrow window, the plasma must be separated and sometimes immediately frozen at very low temperatures to preserve the viability of the coagulation factors for later analysis.
Blood Gas Analysis
Blood gas analysis represents the most time-sensitive category, as the balance of gases and pH in the sample changes rapidly. These samples often require analysis within 15 to 30 minutes. If they are not immediately placed on ice to slow down cellular metabolism, the process alters the measured pH and oxygen levels.
Maximizing Sample Viability Through Proper Handling
Beyond the initial collection, several post-draw handling steps can significantly delay degradation and extend the time a sample remains viable. Temperature control is a primary factor; refrigeration often slows down metabolic processes like glycolysis and the breakdown of certain enzymes. However, some tests, such as those for cold agglutinins, require a specific temperature, and refrigeration can sometimes induce hemolysis in sensitive red blood cells.
Centrifugation is a powerful tool because it physically separates the plasma or serum from the cellular components. This separation stops the cells from consuming analytes like glucose and releasing intracellular components, essentially pausing the most destructive degradation pathways.
For specific analytes, such as bilirubin, protection from light is necessary immediately after collection, as exposure can cause photochemical degradation and lead to falsely low results. Proper handling involves a combination of time management, temperature regulation, and physical separation to preserve the sample’s initial integrity.