1. Mislabelling

2. Air (bubble) in the sample. Ideally, anaerobic collection, i.e., no exposure to air.

3. Appropriate volume requirement

4. Delay in analysis

5. Incorrect Transport and/or Storage

6. Insufficient Mixing (leads to clotted specimen)

7. Wrong collection containers

   • Concern here is insufficient heparin or dilution from liquid heparin

8. No arterial line flush, or insufficient removal of flush solution

9. Venous puncture during arterial sampling

   • Important to identify arterial vs. venous vs. mixed venous for some analytes (e.g. pO2).

• CO2 makes up 0.0407% of atmospheric air

  • Therefore, using Dalton's Law, pCO2 component of 𝑡ℎ𝑒 𝑎𝑖𝑟 = 760 mm Hg X 0.0407% = 0.309 mm Hg

• pCO2  in the arterial blood ~ 40 mm Hg

• Thus air leads to artificial decrease in blood pCO2 (movement of gas from area of high concentration (blood) to area of low concentration(i.e., exposed air)

• pH will increase (blood pH is partially a function of pCO2)

• O2 makes up 20.946% of atmospheric air

• pO2in the air = 760 mm Hg X 0.20946% = 159.19 mm Hg

• pO2 in the arterial blood ~ 80 - 100 mM Hg

• pO2 in the venous blood ~ 30 – 50 mm Hg

• Air leads to artificial increase in blood (movement of gas from area of high concentration (air) to area of low concentration(i.e., blood)

• pO2 increases if initial pO2 is less than that of ambient air (i.e. ~ 150 mmHg, 20kPa), and decreases if initial pO2 is greater than that of ambient air (the latter case would only, of course, apply if the patient was receiving supplemental oxygen).

1. Under-filling alters the intended the 𝐴𝑛𝑡𝑖𝑐𝑜𝑎𝑔𝑢𝑙𝑎𝑛𝑡:𝐵𝑙𝑜𝑜𝑑  ratio

2. Syringes are electrolyte balanced for an expected volume

3. Prevent NSQ for analysis (NSQ = Not sufficient quantity)

Note: While heparin is the anti-coagulant, heparin binds positively charged electrolytes (Na+ and iCa2+)

1. Consumption of O2 by cells; Red blood cells use little or no O2, but white blood cells do consume O2 [1,2]. 

   •  What happens to  values in patients with myeloproliferative diseases with extreme leukocytosis or thrombocytosis. e.g. AML (Acute Myelogenous Leukemia)?

     • Spuriously low pO2 are possible, where the energy intensive WBC will consume O2.  So blood gas analysis should be done as soon as possible, especially in these patients.

2. Increased gas exchange (i.e. loss of O2) through plastic walls [3].

   • Glass syringes preserve pO2 values better than plastic syringes (plastic syringes/containers are more common today, but have high oxygen permeability). Plastic syringes are used today because they are inexpensive, disposable, and not prone to breakage/shatter.

   • Generally, the gas loss through plastic tubes is insignificant, if performed <40 minutes [3].

     • Glass and plastic syringes are ~equivalent for pH and pCO2

1. Kelman, G.; Nunn, J. Journal of Applied Physiology 1966, 21, 1484–1490.
2. Severinghaus, J. W.; Astrup, P. B. Journal of clinical monitoring 1986, 2, 174–189.
3. Zavorsky, G. S.; Wijk, X. M. R. van. Clin. Chem. Lab. Med. 2023, 1–10.

• Ice increases oxygen permeability in plastic tubes, leading to artificial increase in pO2.

• Potassium is also subject to increase with prolonged (>1 hour) storage/transport on ice [1]

  • Why? 

    • Lowered temperatures inhibits red blood cells glycolysis, leading to decreased ATP production, which slows down the Na+/K+ ATPase pump that keeps the potassium inside the cells. This leads to a “leaky” situation, (i.e. redistribution) , where more K+ shifts from the intracellular fluid compartment to the extracellular fluid (plasma) compartment, thus increasing plasma potassium [1].

1. Zavorsky, G. S.; Wijk, X. M. van; Gasparyan, S.; Stollenwerk, N. S.; Brooks, R. A. The Journal of Applied Laboratory Medicine 2022, 7, 541–554.

• Depends on the stability of the pneumatic transport system (PTS).

• In some cases, experience and data suggests most PTS show no significant effect on pH and pCO2 [1]. However, pO2 can be affected if the sample is contaminated with the even tiny amounts of air.

• You will need to verify your own settings and continuously monitor for changes to showcase that equivalent and consistent results can be obtained by transport via PTS.
  
  

1. Pupek, A.; Matthewson, B.; Whitman, E.; Fullarton, R.; Chen, Y. Clinical Chemistry and Laboratory Medicine (CCLM) 2017, 55, 1537–1544.

• Not for pO2or Oxygen saturation. General consensus and guidances from Clinical Laboratory Standards Institute recommend arterial blood gas samples be collected in syringes directly.

• For venous samples, evacuated tubes can potentially be used for pCO2, pH and other measurands, if you have data to support sufficiently equivalent results can be obtained. However, evacuated tubes are not completely vacuum; there is small amount of gas or residual air that can contaminate the results. There is varying practices on the use evacuated tubes (vacutainers). However, data suggests vacutainers can lead to falsely high pH, pO2 , and falsely low pCO2 [1].

Note: Under-filled vacutainers create headspace or a void above the solution, enabling an equilibration between solution and gas phase, which would amplify the errors further [1].

1. Tang, N.-Y.; Leanse, J. H.; Tesic, V.; Wijk, X. M. van. Clinica Chimica Acta 2020, 510, 671–674.

1. Specimen was likely not handled according to standard process

2. Old/Aged specimen

3. Clogs analyzer

4. Clotting can have effects leading to measurement error. 

Are results reliable from clotted specimens?

4. • No. Filters or “clot catchers” are useful and help to reduce analysis of clotted specimens, but do not completely eliminate all clotted specimens. And some instruments have built in clot detection [1].

1. D’Orazio, P. Point Care 2011, 10, 186–188.

• Spectrophotometric assessment of Hemoglobin (Hb) content. In other words, an indirect assessment of pO2 (given by the Hemoglobin-Oxygen Dissociation (HOD) curve)

1. Pulse Oximetry

2. Co-oximetry

3. estimated Oxygen Saturation (eO2 Sat)

The pulse oximeter estimates the oxygen saturation transcutatenously.  By shining light (red and infrared light) at the fingertip or ear lobe and detecting appropriate signal absorption pattern, oximetric parameters are obtained that are related (and calibrated) to measurements of arterial blood oxygen saturation[1].  

This method of assessing oxygen saturation makes several assumptions, what these assumptions are will be asked and discussed subsequently.

1. Keller MD, Harrison-Smith B, Patil C, Arefin MS. Skin colour affects the accuracy of medical oxygen sensors. Nature [Internet]. 2022 Oct;610(7932):449–51. Available from: https://www.nature.com/articles/d41586-022-03161-1

• If there are suspected dyshemoglobins, do not use pulse ox to measure SO2.

  • E.g. a comatosed patients with 15% COHb and by pulse oximetry SO2 ~95% can be achieved, which is inaccurate