The reliability of diagnostic testing depends heavily on the stability of the blood sample. Once blood leaves the body, its biological and chemical environment changes, directly impacting the accuracy of analysis. Determining “how long blood is good for testing” involves a complex matrix of time, temperature, and the specific test required. Clinical laboratories follow strict protocols to manage the time between collection and analysis for every specimen. This discussion focuses exclusively on clinical samples used for diagnostic purposes.
The Biological Basis of Sample Degradation
A blood sample is a living system outside the body, where cellular components continue metabolic processes. The most significant change is the consumption of glucose, called glycolysis, primarily by red and white blood cells. If processing is delayed, glucose levels can drop by an estimated 5% to 7% per hour at room temperature, leading to falsely low results.
Another major source of degradation is cellular rupture, or hemolysis, which releases the contents of red blood cells into the surrounding plasma or serum. Red blood cells contain high concentrations of substances like potassium, lactate dehydrogenase (LDH), and aspartate aminotransferase (AST). When cell walls break down, these components flood the sample fluid, causing falsely elevated test results for these analytes. Degradation is accelerated by mechanical trauma during collection or by prolonged contact between the cells and the plasma or serum. For this reason, samples must be separated quickly.
Stability Requirements for Common Test Categories
The maximum acceptable analysis time varies significantly based on the specific test and the analyte’s sensitivity. Tests measuring cellular components or highly labile substances have the shortest timeframes.
Hematology Tests
Hematology tests, such as the Complete Blood Count (CBC), measure the number and size of blood cells and are performed on whole blood. While red blood cell and hemoglobin counts remain relatively stable for up to 24 hours at room temperature, platelet (PLT) counts and their indices, like Mean Platelet Volume (MPV), are much more sensitive. Platelet analysis is often considered accurate only if performed within six hours of collection.
Chemistry Panels
Chemistry panels assess electrolytes, enzymes, and metabolites, and are highly susceptible to the effects of cellular metabolism. Due to the rapid consumption of glucose and the leakage of potassium, samples intended for these tests must be centrifuged to separate the cells from the serum or plasma within two to four hours of collection at room temperature. Once separated, stability for many other chemistry analytes, such as cholesterol or liver enzymes, can extend up to 24 hours or more.
Coagulation Studies
Coagulation studies, like the Prothrombin Time (PT) and Activated Partial Thromboplastin Time (aPTT), measure the blood’s ability to clot. The PT is relatively stable and remains accurate for up to 24 hours when stored at room temperature or refrigerated. The aPTT is more fragile because certain clotting factors degrade quickly. This test typically requires analysis within four hours of collection, a window that may be shorter for patients on anticoagulant therapies.
How Storage Temperature Affects Viability
Temperature is the primary tool laboratories use to slow biological and chemical degradation.
Room Temperature (20–25°C)
Storing whole blood at room temperature is the least stable condition, and the stability clock for sensitive tests begins immediately upon collection. This temperature accelerates metabolic processes like glycolysis and the breakdown of certain enzymes.
Refrigeration (2–8°C)
Refrigeration extends the stability of many analytes by significantly slowing metabolic activity. Most CBC parameters, for instance, remain reliable for up to 48 hours or longer when refrigerated, compared to the 24-hour limit at room temperature. However, refrigeration can be detrimental to the accuracy of cell morphology analysis, such as that required for a manual differential on a blood smear. Extended cold storage can induce visible changes in cell shape, such as red blood cell crenation or white blood cell vacuolization, which can be misinterpreted as a pathological finding.
Freezing (-20°C or colder)
Freezing is the preferred method for long-term storage of separated plasma or serum. It halts almost all cellular and enzymatic degradation, preserving analyte concentration for months or years. For freezing to be effective, the sample must first be centrifuged, and the plasma or serum separated from the cells without delay. Freezing whole blood is generally avoided because the ice crystals that form will rupture the cells, causing severe hemolysis and rendering the sample unsuitable for most diagnostic tests.
Laboratory Protocols for Delayed and Compromised Samples
Clinical laboratories maintain strict quality control procedures to prevent inaccurate results from compromised samples. The two most common reasons for sample rejection are clotting and hemolysis. Clotting prevents accurate complete blood counts because the instrument cannot count clustered cells.
For chemistry tests, automated analyzers detect and flag samples that are hemolyzed, icteric (jaundiced), or lipemic (cloudy with fat) using a photometric measurement called the Hemolysis Index (HI). Hemolysis is the most frequent issue, and labs often use a specific threshold, such as “moderate” or “gross” hemolysis, that necessitates rejection and recollection.
If a sample exceeds stability limits or shows compromise, but recollection is impossible or would dangerously delay patient care, the laboratory may still proceed. In these cases, the lab issues results with a cautionary comment indicating potential inaccuracy, such as noting falsely decreased glucose due to delayed processing. This ensures the interpreting physician is aware of the data’s pre-analytical limitations.