The time a blood sample can remain in a tube before testing, known as sample stability, is a primary factor determining the accuracy of laboratory results. This period is a critical component of the pre-analytical phase of testing, which includes everything that happens from the moment blood is drawn until it is analyzed. Blood is a complex living tissue, and once removed from the body, its components immediately begin to change. This process can alter the concentration of measured substances, or analytes. Understanding the limits of this stability is necessary for ensuring that laboratory results reliably reflect a patient’s true physiological state. The integrity of the sample depends on its internal chemistry and the stabilizing agents within the collection tube.
Why Time Matters: The Chemistry of Blood Decay
Blood does not become inert when it enters the collection tube; the cells continue their metabolic functions, consuming and releasing substances that can skew test results. One immediate change is the consumption of glucose, a process known as glycolysis, primarily driven by red and white blood cells. If a sample sits unseparated from the cells for too long, the glucose level will artificially decrease, leading to an inaccurate result, particularly noticeable in plain serum tubes.
Cellular activity also leads to the production of metabolic byproducts, such as lactate, which increases over time as glucose is consumed. Prolonged contact between the liquid portion of the blood (serum or plasma) and the blood cells also risks cellular degradation, or hemolysis. When red blood cells break open, they release high concentrations of their internal contents into the surrounding fluid.
This leakage of intracellular components is particularly problematic for analytes concentrated inside the cells, such as potassium, phosphate, and the enzyme lactate dehydrogenase (LDH). An elevated result for any of these tests in a delayed or hemolyzed sample may simply be an artifact of poor sample handling, not a true indicator of a patient’s health. Certain sensitive molecules, including some hormones, enzymes, and trace elements like zinc, also naturally degrade or change their chemical form over time. These combined biological and chemical processes establish the strict time limits laboratories must enforce.
Understanding Blood Collection Tubes
The precise amount of time a blood sample remains stable is heavily influenced by the specific additives inside the collection tube, which is why tubes are color-coded. Many tubes contain anticoagulants, chemicals that prevent the blood from clotting, allowing the sample to be analyzed as whole blood or plasma.
- Anticoagulants like Ethylenediaminetetraacetic acid (EDTA) in lavender-top tubes bind to calcium ions, a necessary cofactor in the clotting cascade.
- Heparin, found in green-top tubes, blocks clotting by inhibiting the enzyme thrombin. These tubes yield plasma after centrifugation, which is often preferred for chemistry panels.
- Sodium citrate, used in light-blue top tubes for coagulation studies, is a reversible anticoagulant that also binds calcium.
Other tubes are designed to yield serum, the fluid portion remaining after clotting has occurred. Red-top tubes may contain a clot activator to speed up this process. Gold or speckled-top tubes often contain both a clot activator and a separation gel. This thixotropic material forms a stable barrier between the serum and the blood cells after the tube is spun down. These additives chemically stabilize the sample, extending the window of time for testing.
Time Limits for Common Laboratory Tests
The stability period varies significantly depending on the test being performed and the tube type used.
Hematology (CBC)
For a Complete Blood Count (CBC) in an EDTA tube, the cellular components are generally stable for 24 hours when stored at room temperature. Refrigeration can extend the stability of most parameters for up to 48 hours, though prolonged storage can alter the appearance of white blood cell morphology.
Chemistry Panels
Routine chemistry and metabolic panels have analytes with widely different stabilities. Highly sensitive analytes, such as ammonia and lactate, require immediate processing, often within 15 to 30 minutes, and may necessitate collection in special tubes and transport on ice. Most common chemistry analytes, like sodium and creatinine, are stable for 48 to 72 hours if the plasma or serum is separated from the cells. Glucose is a major exception: if plasma or serum is not separated from the metabolically active cells within one to two hours, the concentration will drop rapidly.
Coagulation Tests
Coagulation tests, such as Prothrombin Time (PT) and Activated Partial Thromboplastin Time (aPTT), are particularly time-sensitive as the clotting factors begin to degrade quickly. Unseparated blood in a sodium citrate tube is typically stable for a maximum of four hours at room temperature. Some routine tests like PT/INR may be stable for up to 24 hours if the sample is maintained at a specific temperature. Exceeding these limits can lead to the rejection of the sample by the laboratory.
Best Practices for Sample Handling
Maximizing the time a blood sample remains viable relies heavily on proper handling immediately after collection. Temperature control is a fundamental practice, as chemical reactions accelerate at warmer temperatures. Refrigeration at 2 to 8 degrees Celsius is often necessary to slow down metabolic and degradation processes for longer-term storage.
For time-sensitive tests like ammonia, the sample must be placed in a slurry of ice immediately after collection to halt metabolic activity. Centrifugation is a critical step for many tests, separating the liquid plasma or serum from the blood cells. This separation must occur promptly, often within two hours of collection, to prevent the exchange of substances between the cells and the fluid.
Once separated, the plasma or serum can be transferred to a secondary tube and stored for much longer periods, sometimes days or weeks, depending on the test. Proper transport also plays a role in stability; excessive shaking or vibration can cause physical damage to the red blood cells, leading to artificial hemolysis. Samples for light-sensitive analytes, such as bilirubin, must be protected from light exposure during all phases of handling.