The integrity of a blood sample is determined by how closely its chemical composition mirrors the patient’s state at the moment of the draw. Controlling variables after the blood leaves the vein is necessary to guarantee accurate laboratory results. Temperature is a fundamental control point because biological processes within the blood do not stop once the sample is collected. Maintaining the proper storage temperature prevents the degradation or alteration of sensitive substances, ensuring the test results truly reflect the patient’s physiology.
Analytes That Demand Immediate Ice
Certain blood components, known as analytes, are inherently unstable and require immediate chilling to prevent rapid breakdown or transformation. One common test requiring this special handling is the measurement of lactic acid, often collected in a gray-top or heparin tube. Failure to promptly place the lactic acid sample on ice allows blood cells to continue metabolic activity, rapidly converting pyruvate into lactate, leading to a falsely elevated result. The determination of ammonia levels also requires immediate cooling, typically using a green-top tube with heparin, because ammonia is highly volatile and its concentration can quickly change at room temperature.
The body’s stress hormones, known as catecholamines (including plasma metanephrines), must be collected and chilled immediately to stabilize them. These substances are sensitive to heat and enzyme activity, and cooling slows the degradation process that would otherwise result in falsely low measurements. Tests for Renin Activity and Angiotensin Converting Enzyme (ACE) are also often collected in chilled tubes and placed on ice immediately after the draw. This cooling preserves the enzyme structure and activity, ensuring the measured concentration accurately reflects the body’s status.
Other sensitive tests include Pyruvate, which is closely linked to lactic acid metabolism, and parathyroid hormone-related peptide (PTHrP). Pyruvate is often collected in an EDTA tube and chilled to stop cellular consumption and conversion. The immediate drop in temperature acts as a chemical brake, preserving the analyte concentration until the laboratory can process the sample.
Understanding Sample Instability
The need for chilling sensitive samples stems from the ongoing biological and chemical activity within the collected blood. Even outside the body, blood cells continue cellular metabolism, consuming nutrients and producing waste products. This is particularly evident in glycolysis, where blood cells convert glucose into pyruvate and then lactate. If a sample remains at room temperature, this cellular metabolism continues unabated, leading to an artificially high test result.
Cooling the tube to a temperature near 0°C dramatically slows the rate of these metabolic reactions. This reduction in kinetic energy inhibits the enzymes responsible for glycolysis and other cellular processes, minimizing the change in analyte concentration over time. Instability also involves enzyme activity, where specific enzymes naturally present in the blood plasma continue to break down target molecules.
Peptide hormones and volatile compounds are highly susceptible to enzymatic degradation or rapid chemical change at ambient temperatures. The lowered temperature reduces the efficiency of these degradation enzymes, preserving the integrity of the target analyte. The cooling step is a necessary intervention to halt these natural post-collection processes before the sample can be analyzed.
Essential Steps for Cooling and Transport
The effectiveness of chilling a sample relies heavily on the proper technique used immediately following the blood draw. The most effective method for immediate cooling is placing the blood tube into an ice slurry, rather than simply cubed ice. An ice slurry, a mixture of crushed ice and water, ensures the tube is completely surrounded, maximizing contact with the cold surface and providing rapid, consistent cooling. The tube must be placed in this slurry immediately after collection and gentle mixing, typically within seconds, to arrest cellular and enzymatic activities.
During transport, the sample must remain in the chilled environment to ensure the preservation of the analytes until it reaches the testing instrument. The tube is secured within a container surrounded by the ice slurry, ensuring the cold chain is maintained without interruption. The transport container should be clearly labeled and handled gently to prevent hemolysis, which can release intracellular components and interfere with test results.
Exceptions to Chilling
Not all blood samples should be chilled; some require room temperature storage. Tubes collected for certain chemistry tests, especially those with gel-barrier separators, should not be placed directly on ice. Freezing can interfere with the gel’s ability to create a stable barrier between the serum or plasma and the blood cells during centrifugation, compromising sample integrity. Additionally, unspun whole blood samples intended for potassium testing should not be refrigerated, as cooling can cause potassium to leak out of the red blood cells, leading to a falsely elevated result.