I am a supervisor of a health and wellness center and am responsible for collating lab reports on specimens sent to a reference laboratory. On some reports, results are marked as NSA (not suitable for analysis)—for example, glucose levels on specimens collected in SST tubes. What are some reasons why this would occur?
It has been reported that up to 70 percent of laboratory medical errors occur during the pre-analytical phase of patient testing.1 The majority of errors are due to physical issues such as ordering the wrong test, improper preparation of the patient, mislabeling the blood collection tubes, or drawing blood from the wrong patient. Most laboratories have attempted to resolve these issues through specific quality management practices.
Additionally, some laboratory results can vary due to physiological factors such as exercise, diet, stress, posture, age, and gender.2 Other factors that influence results may include tobacco use, alcohol abuse, and serum/plasma samples that are turbid (hyperlipemia), icteric (elevated bilirubin), hemolyzed (poor blood collection technique), short-draws, and clotted tubes.1,2
Another source of potential error that has been identified is more of a mechanical issue and is rarely considered a problem: the type of blood collection tube. Yet a number of studies have shown that the type of tube used for collection and its components (tube wall, rubber stopper, surfactant, anticoagulant, separator gels, and clot activators) may influence test results.3-6
Prior to 1990, glass collection tubes made from soda-lime or borosilicate materials were commonly used. It was found in some instances that certain trace elements leached into blood samples, potentially altering true results.3 Blood collection tubes were subsequently made from plastics, such as polyethylene terephthalate (PET) or polyethylene/polypropylene (PP), thus minimizing breakage and exposure to biohazards and enabling better resistance to shock impacts and enhanced tolerance to centrifugation speeds.6 One such tube that is used in most laboratories is the serum-separator tube (SST).
Some technical analysis interferences have been observed with certain assays that may have been affected by the serum separation collection tube itself. Reported analytes include assays for avidin-biotin immunoradiometric testing (thyrotropin, prolactin, HCG), triidothyronine, vitamin B12, C-reactive protein, various drugs (phenytoin, phenobarbitol, carbamazepine, quinidine, and lidocaine), myoglobin, CK-MB, and glucose.3-6
Serum separation tubes use a gel material with a clot activator that forms a barrier between serum and red blood cells. The clot activator consists of silica particles that coat the wall of the tube, which has to be mixed thoroughly for activation.
Recommended collection process is to invert the tube five (5) times (180o inversion and return X 5), allow to sit for thirty (30) minutes, and then centrifuge at 1,000-1,300 relative centrifugal force (RCF) in a swing bucket for ten (10) minutes.7 An SST specimen should be centrifuged within two (2) hours after collection.7
To achieve accurate test results, good blood collection technique is essential. This means performing a clean phlebotomy and drawing tubes in the proper order of anticoagulant: blood culture, sodium citrate (blue), SST (red/gray or gold), non-additive tube (red), heparin (green), EDTA (lavender), and ACD (yellow).8 Additionally, the tube must be filled to its designated volume in an upright position, inverted completely and the proper number of times, and centrifuged in a timely fashion.
In addressing the specific query regarding glucose testing, more information is needed to resolve specific events (physical, physiological, or mechanical) that may contribute to “unsuitable” specimens.
However, it should be noted that in a recent, small study, blood was collected in three types of tubes (red-top serum separator tubes, gray-top fluoride tubes, and green-top heparin tubes) and measured for glucose.5 The investigators found that the least variation in results occurred with the red-top separator tubes. Studies by one manufacturer showed that there were no clinically significant differences when comparing collection tubes for most routine chemistry tests.9,10
Manufacturers of blood collection tubes have done their best to minimize potential interfering materials from collection tubes that may be incorporated during the blood collection. Yet laboratorians should be aware that spurious interferences may be present and that each laboratory is ultimately responsible for evaluating equipment and developing normal reference ranges. Careful evaluation of how specimens are collected, centrifuged, stored, and transported should be considered. Further, open communication between the facility staff and the laboratory staff should be pursued to ensure best patient outcomes, minimize unnecessary costs, limit the need to redraw patients, improve laboratory productivity, and decrease testing turnaround time.
- Plebani, Mario. Pre-analytical errors and patient safety/Preanaliticˇke greške i bezbednost pacijenata. J Med Biochem. 2012;31(4):265-270.
- Lifshitz MS. Preanalysis, in Henry’s Clinical Diagnosis and Management by Laboratory Methods, 23ed ed. McPherson and Pincus, eds. Elsevier: St. Louis. 2016:20-32.
- Bowen RAR, Hortin GL, Csako G, et al. Impact of blood collection devices on clinical chemistry assays. Clin Biochem. 2010;43:4-25.
- Bowen RAR, Chan Y, Cohen J, et al. Effect of blood collection tubes on total triiodothyronine and other laboratory assays. Clin Chem. 2005;51(2):424-433.
- Geling L, Cabanero M, Wang Z, et al. Comparison of glucose determinations on blood samples collected in three types of tubes. Ann Clin Lab Sci. 2013;43(3):278-284.
- Bowen RAR, Remaley AT. Interferences from blood collection tube components on clinical chemistry assays. Biochemia Medica. 2014;24(1):31–44.
- Becton Dickinson. Product FAQs. http://www.bd.com/vacutainer/faqs/.
- University of Texas Medical Department. Specimen collection. https://www.utmb.edu/lsg/Pages/SpecimenCollection/SpecCollErrors.aspx.
- Becton Dickinson. Comparison of BD Vacutainer Serum, SST and PST Tubes for Common Chemistry Analytes. 2002. https://paswhitepapers.bd.com/.
- Becton Dickinson. Evaluation of BD Vacutainer SST Plus Tubes Compared to BD Vacutainer SST Glass Tubes for Routine Chemistry Analytes. 2004. https://paswhitepapers.bd.com/.
Editor’s note: Anthony Kurec, MS, H(ASCP)DLM, is Clinical Associate Professor, Emeritus, at SUNY Upstate Medical University in Syracuse, NY.He is also a member of the MLO Editorial Advisory Board.