Why Do We Do What We Do: Common Hospital Practices Revealed

By Dana Zalkin

Peer Reviewed

A code is called on the overhead speaker and the on-call teams rush to the scene to see what awaits them. EKG leads are being placed, medications are being ordered, and labs are being drawn. A medical student stands with a bag of ice, ready to grab the arterial blood gas (ABG) and run it down to the lab. “Why do we put the ABG on ice right away?” the student wonders. But in this moment, while a patient teeters on the border of life and death, it seems inappropriate to ask this simple question, one that can always wait till later.

Every day in the hospital great questions arise that may at times seem trivial compared to the enormous mission of taking care of patients or compared to the overwhelming amount of medical knowledge that students and residents constantly try to amass. However, to be the best physicians we can be, it is important to answer these questions and to know: why do we do what we do, and what is the evidence for it?

Q: Why do we put an ABG on ice immediately?

A: Arterial blood gas collection is a crucial step in determining the acid-base status of a patient as well as in evaluating ventilation and gas exchange. As ABGs are commonly used in emergency departments and ICUs, it is imperative that the values be obtained quickly and accurately. Many studies have been done to assess the effects of storage of blood in different syringe types, over various periods of time, and at differing temperatures. Since as early as the 1970s, investigators have assessed the differences that arise from storing blood in glass versus plastic syringes. A study from 1971 published in BMJ demonstrated much greater changes in oxygen tension over time for samples stored in each of 5 different models of plastic syringes as compared to a glass syringe [1]. Another study assessing blood gas results based on syringe type demonstrated that samples stored in glass syringes could provide adequate results over a much longer storage time period than those stored in plastic syringes [2]. However, it is suggested that despite these results, it is not practical to use glass syringes in all clinical situations and that clinical decision-making may not even be influenced by the variation in measurements obtained.

Some data suggests that if analysis of a sample is delayed for even ten minutes at room temperature, PaO2 values will be significantly lowered due to continued consumption of oxygen by the leukocytes and platelets in the sample [3, 4]. Other sources demonstrate that PaO2 values are actually significantly higher when analysis was delayed. For example, one study showed that when samples were stored in plastic syringes for 30 min at 22°C the PaO2 increased by 11.9mmHg compared to immediate analysis or storage in a glass syringe [5]. Theories to explain the increase in PaO2 include the presence of air bubbles in the samples as well as diffusion of gas through the pores of the syringe [5, 6].

Finally, there is a significant amount of data analyzing the practice of placing samples on ice or keeping them at room temperature prior to analysis. With regard to the theory that cellular metabolism reduces the PaO2 over time in samples, some of the literature has suggested that if the sample is put on ice immediately, the metabolic activity of these cells is reduced and they will consume less oxygen [4, 7]. However, many studies have shown increases in PaO2 in samples over time, necessitating an alternate explanation of these changes (two of which are described above) and additional analysis of the effect of cooling samples. Two different studies showed greater increases in PaO2 when plastic syringes were stored at 0-4°C compared to 22°C (8.4-13.7mmHg versus 2.6-11.9mmHg increases at 0-4°C compared to 22°C, respectively) [5, 8]. One theory to explain the increase in PaO2 in samples collected in plastic syringes and cooled postulates that the plastic molecules contract when cooled which subsequently opens larger pores for oxygen to diffuse through, thereby falsely elevating the PaO2 [9]. So, what is the bottom line? After reviewing all the data, we are left more confused than when we started. Nevertheless, the one practice all these studies agree on is the need for rapid analysis of the samples. So whether you put it on ice or keep it at room temperature, bringing that syringe to the lab as fast as possible is sure to yield the best results.

Q: Why do we have to fill the blue-top coagulation tube to the top?

A: You finish morning rounds and begin looking at your long list of things to do that afternoon. One patient needs additional labs done as soon as possible, so instead of calling phlebotomy, you do the blood draw yourself. After anxiously awaiting the results, you check the computer and see “insufficient sample” under the coagulation panel. Heartbreak ensues. The root of this unfortunate tale is the very specific ratio of sodium citrate in the collection tube to blood that is collected. This ratio of 1:9 sodium citrate/blood exists to prevent coagulation of the blood sample; the citrate ions in the tube chelate the calcium in the sample and form calcium citrate complexes, thereby preventing the clotting mechanism [16, 17]. If a sample is “insufficient”, there will be excess anticoagulant in relation to sample and the results will be falsely prolonged. So, the next time you’re drawing blood in a coagulation tube, make sure not to skimp out on the blood. Understanding the reasoning behind what we do: improving efficiency and preventing heartbreak.

Dana Zalkin is a 4th year medical student at NYU Langone Medical Center

Peer Reviewed by Neil Shapiro, Editor-In-Chief, Clinical Correlations

Image courtesy of Wikimedia Commons

References:

1) Scott PV, Horton JN, Mapleson WW. Leakage of oxygen from blood and water samples stored in plastic and glass syringes. Br Med J. 1971 Aug 26;3(5773):512-6.  http://www.ncbi.nlm.nih.gov/pubmed/5565518

2) Picandet V, Jeanneret S, Lavoie JP. Effects of syringe type and storage temperature on results of blood gas analysis in arterial blood of horses. J Vet Intern Med. 2007 May-Jun;21(3):476-81.

3) Trulock EP III. Arterial Blood Gases. In: Walker HK, Hall WD, Hurst JW, editors. Clinical Methods: The History, Physical, and Laboratory Examinations. 3rd edition. Boston: Butterworths; 1990. Chapter 49.

4) Verma AK, Paul R. The interpretation of Arterial Blood Gases. Australian Prescriber. 2010;33:124–9.  http://www.australianprescriber.com/magazine/33/4/124/9

5) Knowles TP, Mullin RA, Hunter JA, Douce FH. Effects of syringe material, sample storage time, and temperature on blood gases and oxygen saturation in arterialized human blood samples. Respir Care. 2006 Jul;51(7):732-6.

6) Lu JY, Kao JT, Chien TI, Lee TF, Tsai KS. Effects of air bubbles and tube transportation on blood oxygen tension in arterial blood gas analysis. J Formos Med Assoc. 2003 Apr;102(4):246-9.  http://www.ncbi.nlm.nih.gov/pubmed/12833188

7) Schmidt C, M?ºller-Plathe O. Stability of pO2, pCO2 and pH in heparinized whole blood samples: influence of storage temperature with regard to leukocyte count and syringe material. Eur J Clin Chem Clin Biochem. 1992 Nov;30(11):767-73.

8) Mahoney JJ, Harvey JA, Wong RJ, Van Kessel AL. Changes in oxygen measurements when whole blood is stored in iced plastic or glass syringes. Clin Chem. 1991 Jul;37(7):1244-8.

9) Beaulieu M, Lapointe Y, Vinet B. Stability of PO2, PCO2, and pH in fresh blood samples stored in a plastic syringe with low heparin in relation to various blood-gas and hematological parameters. Clin Biochem. 1999 Mar;32(2):101-7.    http://www.ncbi.nlm.nih.gov/pubmed/10211625

16) “Buffered Sodium Citrate 3.2% (0.109M)(100 Ml (10 Pouches)).” Aniara. N.p., n.d. Web. 12 July 2014. http://www.aniara.com/PROD/A12-8480-10.html.

17) “Lab Manual for UCSF Clinical Laboratories.” UCSF Departments of Pathology and Laboratory Medicine. N.p., n.d. Web. 12 July 2014. http://labmed.ucsf.edu/sfghlab/test/CoagulationProcedures.html.

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