Note: The comments in this discussion relate to blood and blood derivatives. Other types of sample will be addressed in a later topic in the “In Focus” section of this website
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Stability has been defined by the International Standards Organisation (ISO) as the capability of a sample material to retain the initial property of a measured constituent for a period of time within specified limits when the sample is stored under defined conditions (ISO Guide 30, 1992). Instability is present when there are important changes in one or more of those measurements.
Even before a collection tube is filled with blood, the empty tubes must be stored according to manufacturers instructions. Failure to comply with these can influence the stability of the blood sample that is subsequently collected into that tube.
It is important to keep in mind that sample transport and storage conditions, together with the time interval between collection and testing, can have an important effect on the quality of test results.
Despite increased point-of-care testing in many areas of laboratory medicine, the vast majority of specimens are collected in one place and transported to another for analysis. Some tests are available only in certain reference centres, requiring samples to be transported over long distances.
Further, in many countries, testing is increasingly consolidated in fewer but larger laboratory facilities with more and more centralisation of the pathology laboratory service. This centralisation has increased the focus on controlling variabilities around the sample transport and storage aspects of the preanalytical phase. Consideration of the effects of sample transport and storage1 and requiring evidence of sample stability are particularly important, since this pivotal aspect of lab testing is not normally assessed by proficiency testing programmes.
A number of studies have carefully evaluated the changes in test results which may occur over time and which are influenced by storage conditions2. These studies may detail statistically significant changes in test results over time, but it is important to take into account whether these differences are clinically relevant. Small differences may be tolerated if there is clearly no impact on patient management.
Data relating to sample stability may also depend on the tube type3 used for blood collection, (including any separation gels, anticoagulants and other additives present) the temperature of storage prior to testing, and the laboratory method3 used for determination. This is particularly true in relation to hemostasis.
The mode of transporting samples to the laboratory may be relevant, as well. Rapid sample delivery via pneumatic tube transportation is attractive for reducing transport times and is an acceptable method of sample transport for some types of laboratory tests. In some cases, however, such as blood gas measurement, at least some pneumatic transfer systems4 may be unsuitable. In relation to Blood Gas measurements it is essential that any air bubbles are eliminated form the collection device on collection and prior to transport.
For some lab tests, samples may be stored in the laboratory prior to analysis, when tests are performed in batches for efficiency reasons or when a test is added after completion of the original group of analysis. Such storage conditions5 may critically affect the results obtained, and findings based on one set of study conditions cannot always be safely extrapolated to other conditions. Further, storage temperature options6 such as refrigeration or deep-freezing should not automatically be assumed acceptable without supporting data. Both can induce changes in test results.
Access to data relating to the effects of specific modes of storage and transport is critical. Laboratories should keep control of these important preanalytical variables and use only validated approaches to these aspects of the preanalytical phase.
- CLSI (formerly NCCLS) document H21-A5. Collection, Transport, and Processing of Blood Specimens for Testing Plasma-Based Coagulation Assays and Molecular Hemostasis Assays; Approved Guideline - Fifth Edition (ISBN 1-56238-657-3). Clinical and Laboratory Standards Institute, 940 West Valley Road, Suite 1400, Wayne, Pennsylvania 19087-1898 USA, 2008.
- International Standard Organisation document ISO 15189: 2007. Medical laboratories - Particular requirements for quality and competence.
- Quality and reliability of routine coagulation testing: can we trust that sample? Lippi G, Franchini M, Montagna M, Salvagno GL, Poli G & Guidi GC. Blood Coag Fibrinol 2006; 17(7): 513-519.
Footnote 2: Blood samples used for some molecular genetic tests (e.g., factor V Leiden) are stable for many days over a wide range of ambient temperatures. In contrast, many plasma or serum based measurements can incur deterioration and instability. (Storage of serum or whole blood samples? Effects of time and temperature on 22 serum analytes. Heins M, Heil W & Withold W. Eur J Clin Chem Clin Biochem 1995; 33(4): 231-238).
Footnote 3: Blood samples collected for monitoring oral anticoagulant therapy by determination of Internationalised Normalised Ratio (INR) have been reported to be stable for up to 3 days (Reliability of delayed INR determination: implications for decentralized anticoagulant care with off-site blood sampling. Baglin T & Luddington R. Br J Haematol 1997; 96(3): 431-4). Other authors recommend a maximum of 6 hours between sample collection and testing for INR (Preanalytical variables and off-site blood collection: influences on the results of the prothrombin time/international normalized ratio test and implications for monitoring of oral anticoagulant therapy. Van Geest-Daalderop JH, Mulder AB, Boonman-de Winter LJ, Hoekstra MM & Van den Bessellar AM. Clin Chem 2005; 51(3): 561-8). These authors noted advantages of using a separator gel on the stability of analytes. (Effects of storage temperature and time before centrifugation on ionized calcium in blood collected in plain vacutainer tubes and silicone-separator (SST) tubes. Toffaletti J, Blosser N & Kirvan K. Clin Chem 1984; 30(4): 553-6).
Footnote 4: (Pneumatic transport exacerbates interference of room air contamination in blood gas samples. Astles JR, Lubarsky D, Loun B, Sedor FA & Toffaletti JG. Arch Pathol Lab Med 1996; 120(7): 642-7 and Changes in blood gas samples produced by a pneumatic tube system. Collinson PO, John CM, Gaze DC, Ferrigan LF & Cramp DG. J Clin Pathol 2002; 55(2): 105-7) One study confirmed that transport over 100 metres taking 2 minutes did not induce cell fragmentation, and haematology cell counting parameters were unaffected (Effects of a pneumatic tube system on routine and novel hematology and coagulation parameters in healthy volunteers. Kratz A, Raneem OS & Van Cott EM. Arch Pathol Lab Med 2007; 131(2): 293-6). The same study demonstrate that clotting screen tests were similarly unaffected. Nevertheless, reports indicate that there is potential for additional trauma influence some test results. Important changes induced by pneumatic tube transport were reported by Dyszkiewicz-Korpanty and co-workers (The effect of a pneumatic tube transport system on PFA-100 closure times and whole blood platelet aggregation. Dyszkiewicz-Korpanty A, Quinton R, Yassine J & Sandoc R. J Thormb Haemost 2004; 2(2): 654-6), who demonstrated that platelets can be adversely affected. The concentration of potassium and other intracellular components might also be influenced, in that such compounds might be released upon haemolysis of the specimen when transported through the pneumatic tube system (Pneumatic tube system induced haemolysis: assessing sample type susceptibility to haemolysis. Sodi R, Darn SM & Stott A. Ann Clin Biochem 2004; 41(Pt 3): 237-40). Findings may be specific for a particular pneumatic tube system, sample tube type, lab test, and method employed. If data are not available validating a particular type of sample transport for particular lab test, then the laboratory must consider checking for such effects.
Footnote 5: Adding on an activated partial thromboplastin time (APTT) to a sample for which plasma had been left for 8 hours over spun down blood sample at room temperature was shown to be acceptable for some patient groups (Stability of plasma for add-on PT and APTT tests. Neofotistos D, Orpeza M & Tsao CH. Am J Clin Pathol 1998; 109(6): 758-63). However, this would be completely unacceptable if such a sample were collected from a patient receiving unfractionated heparin therapy, where a substantial component of the activity can be lost in 4 hours of storage.
Footnote 6: Storage of whole blood at 4°C can lead to loss of FVIII, and specially von Willebrand's factor activity to the extend that test results consistent with von Willebrand disease can be obtained on blood from normal subjects (Potential laboratory misdiagnosis of hemophilia and von Willebrand disorder owing to cold activation of blood samples for testing. Favaloro EJ, Soltani S & McDonald J. Am J Clin Pathol 2004; 122(5): 686-92). The following authors demonstrated the stability of a number of chemistry analytes when frozen at -20°C in primary tube with a gel based separator: An evaluation of the integrity of BD Vacutainer® SST™ II and analyte stability when subject to freezing at -20°C. Bakker J, Hackeng C, Church S, Green S, van Dieijen-Visser M & Bekers O. Euregio Congress of Clinical Chemistry and Laboratory Medicine 2003, poster P1.4.
Last modification on 7 July 2012