A blood sample’s viability, or how long it remains representative of a patient’s condition for accurate testing, is governed by strict limits on time and storage conditions. Maintaining the original chemical and biological composition captured at the moment of collection is essential. If the sample’s components begin to degrade or change, the resulting laboratory test data will no longer reflect the patient’s true health status. Adherence to strict pre-analytical protocols, including timely processing, ensures the reliability of medical diagnoses.
Defining Sample Stability
The stability of a blood sample is directly tied to its specific form after collection. Whole blood, which contains all cellular components, is typically used for tests such as a Complete Blood Count (CBC). Because the cells remain active and consume nutrients, whole blood is the least stable form and must be analyzed quickly.
Plasma is the liquid portion obtained when an anticoagulant is added to the tube, preventing clotting. Centrifugation separates the cells from this fluid, which still contains clotting factors like fibrinogen. Serum is collected without an anticoagulant, allowing the blood to clot naturally; the resulting clear fluid lacks fibrinogen. Separated serum and plasma are generally more stable than whole blood because the cells, which are the main source of component breakdown, have been removed.
Environmental Factors Influencing Degradation
Temperature is the most significant environmental factor determining how long a sample remains viable. Heat dramatically accelerates the degradation of blood components, including enzyme activity and the breakdown of cell membranes. Samples left at room temperature are stable for only a few hours before changes in analyte concentration begin. Refrigeration (4°C) is the standard method for short-term preservation, extending stability for days. Freezing (-20°C or colder) is reserved for long-term storage and requires the sample to be separated into serum or plasma beforehand.
Time Limits for Common Laboratory Tests
Different laboratory tests have specific time limits that dictate how long a sample can be stored before analysis. For a Complete Blood Count (CBC), which analyzes cell shape and quantity, the whole blood sample must be tested quickly. Most parameters remain stable for only 24 hours when stored at room temperature. Refrigeration can extend this slightly, but cell morphology changes begin relatively soon, affecting the accuracy of the results.
Glucose testing is one of the most time-sensitive analyses because blood cells continue to metabolize glucose through glycolysis after the blood is drawn. If whole blood is left at room temperature without a specialized preservative, glucose levels can drop by 5% to 7% per hour. To counter this, samples are collected in tubes containing antiglycolytic agents like sodium fluoride, or the plasma must be separated from the cells within one or two hours of collection.
The Basic Metabolic Panel (BMP), which measures electrolytes, kidney function markers, and glucose, has varying stability depending on the specific analyte. If the serum or plasma is separated and refrigerated, many components remain stable for days. However, the overall stability for a comprehensive panel is cited as less than 12 hours at refrigeration temperatures to ensure reliability for all included tests.
Consequences of Testing Degraded Samples
Using a blood sample that has exceeded its stability limit or been improperly stored leads to clinically significant problems. The most common issue is “analyte drift,” where the concentration of the substance being measured changes due to chemical or biological activity within the tube, resulting in false high or false low readings. Another frequent form of degradation is hemolysis, which occurs when red blood cells rupture and release their contents into the plasma or serum. The released contents, such as potassium and enzymes, can artificially elevate measured levels and interfere with testing equipment by changing the sample’s color. If a sample is deemed unstable, the laboratory must reject it, requiring a re-draw from the patient to obtain a fresh specimen.