Preanalytics in the

Analysis of

Arterial Blood Gases

Meqat General Hospital December 2016

By

Prof. Asmaa El Reweny, MD

Professor & Consultant of Clinical & Chemical Pathology, Faculty of Medicine, Cairo University & AMS, Taibah University (2006-2016)

Objectives

By the end of this lecture you will be able to:

1. Recognize why the sample of ABGs

analysis is so special sample ?

2. Identify important precautions before,

during & after sampling for ABGs.

3. Recognize potential errors & how to avoid

them.

2 Prof Asmaa El Reweny, MD

“The weak link”

The preanalytical phase is the

weak link in the Patient Focus

Circle.

Blood gas analyzers of today are

highly accurate.

Make sure that sample

represents patient status.

Many potential errors could be

overcomed by

Training

User guidelines

Sampling products

Aterial Blood Gases Analysis

“Collection of blood, as well as its handling and transport are key

factors in the accuracy of clinical laboratory analysis and ultimately

in delivering quality patient care”

”Arterial blood is one of the most critical specimens sent to the

clinical laboratory”

”Blood gas and pH analysis has an immediate effect on

patient care than any other laboratory test”

”In blood gas and pH analysis an incorrect result

can often be worse for the patient than no result at

all”

4 Prof Asmaa El Reweny, MD

What is so special about blood gases? NOT like other blood samples

STAT Test

Must be analyzed within a

short time

Short Turn Around Time

pO2, pCO2, pH, LAC, GLU

Sample composition changes

Patient status changes

5 Prof Asmaa El Reweny, MD

To get an actual assessment of respiratory

condition, the patient should be in a steady

state of ventilation

Patients should be at rest for 5 min

Ventilation should be stable for 20 min.

Pain and anxiety from arterial puncture may

influence the steady state of respiration

and should thus be minimized

Stabilization of the respiration.

Special Handling of Blood Specimens:

ABG will require chilled tube in order to maintain the stability of the analytes.

A slurry of ice and water is recommended for chilling the tubes of blood.

7 Prof Asmaa El Reweny, MD

Blood Specimen Transport

Specimens for ABG must be transported

immediately .

8 Prof Asmaa El Reweny, MD

Sampling from A-lines

Preparation prior

to sampling

Sampling/

handling

• Label the samples with patient ID. • Use dry electrolyte balanced heparin. • Try to keep patient’s respiration stable for

certain period prior to sampling.

• Make sure that the A-line has been adequately cleared of flush solution.

• Aspirate the sample slowly to prevent bubbles & hemolysis.

• Expel any air bubbles immediately after sampling.

• Mix the sample thoroughly with heparin after sampling.

9 Prof Asmaa El Reweny, MD

Preparation

Prior to

Sample Transfer

•Before transferring sample into the analyzer mix thoroughly. •Visually inspect the sample for clots & air bubbles. •Enter patient ID in analyzer logs.

Storage/

Transport

•Analyze sample immediately. •If storage is unavoidable, store the sample at room temperature for max 30 min (if plastics). Samples with expected high pO2 values should be analyzed within 5 min.

10 Prof Asmaa El Reweny, MD

Storage Recommendations

Storage and transport time

should be kept at a minimum:

Volatile nature of gases

Continued metabolism in

blood.

For parameter panels including

GLU/LAC, be aware that 30

min storage might lead to biased

results.

It is recommended by the

NCCLS to avoid cooling of

samples when kept in plastics.

General storage recommendation:

Do not cool the sample.

Analyze within 30 minutes.

For samples with high pO2:

Analyze within 5 minutes.

For special studies, e.g. shunt:

Analyze within 5 minutes.

For samples with high leucocyte or

platelet count:

Analyze within 5 minutes.

Expected delayed analysis:

When analysis is expected to be delayed

for more than 30 minutes, the use of glass

syringes and storage in ice slurry is

recommended.

pO2 oxygen will still be consumed

pCO2 carbon dioxide will still be produced

pH due to changes in pCO2 and glycolysis

cCa2+ change in pH will influence binding of Ca2+ - to proteins

cGlu glucose will be metabolized

cLac due to glycolysis

Continued cellular metabolism in sample

12 Prof Asmaa El Reweny, MD

Slowing down the metabolism Blood gas samples in glass samplers

could be cooled: storing the sample

at lower temperature (0-4 °C) will

slow down metabolism by at least a

factor of 10 [NCCLS].

Cool samples in an ice slurry or

other suitable coolant.

Never store the samples directly on

ice as this causes hemolysis of blood

cells.

NCCLS Document C27-A; Blood Gas Pre-Analytical Considerations: Specimen Collection, Calibrations and Controls; Approved Guideline

25 C

0-4 C

pO2

Time

13 Prof Asmaa El Reweny, MD

Potential Preanalytical Errors in

ABGs Analysis Preparation

prior to sampling

• Missing or wrong patient/sample identification.

• Wrong type or amount of anticoagulant:

- dilution due to use of liquid heparin

- insufficient amount of heparin.

- binding of electrolytes to heparin.

• Inadequate stabilization of respiration of the patient.

• Inadequate removal of flush solution in A-lines prior to blood collection.

14 Prof Asmaa El Reweny, MD

Sampling/

handling

• Mixing of venous with arterial blood during puncturing

• Air bubbles in the sample • Insufficient mixing with heparin • Incorrect storage • Hemolysis of blood cells

Storage

& transport

Prep prior

to analysis

• Presence of clots • Inadequate mixing of sample

before analysis • No instrument identification of

sample upon analysis 15 Prof Asmaa El Reweny, MD

Mixing venous and arterial blood When puncturing an artery it is

important not to get the arterial blood mixed with venous blood.

This may occur if you hit a vein before locating the artery.

Even an admixture of small amount of venous blood may significantly bias the results.

This is especially true for pO2 and sO2, but other parameters may also be affected

Vein

Artery

40 mmHg / 5.3 kPa

100 mmHg / 13.3 kPa

Mixing venous and arterial blood

In arteries the blood pressure

is high enough to fill a self-

filling syringe

If a self-filling syringe does

not fill, it may be because a

vein has been hit

In that case a new sample

should be taken

Vein:

Pressure rarely > 10 mmHg

Artery:

Systolic blood pressure normally > 100 mmHg

Inadequate removal of flush solution

Flush solutions must be

removed completely from the

system to avoid dilution of the

blood sample

It is recommended to withdraw

a volume equal to 3-6 times the

“dead space” of the catheter

system (NCCLS).

Inadequate removal of flush solutions

Sample B and A are both A-line samples taken from the same patient immediately after each other

Before taking sample B only 1 mL of saline solution was removed - the tubing, however, looked red

Before taking sample A saline solution was removed as recommended

Sample A ctHb 6.2 mmol/L

cGlu 9.6 mmol/L

cK+ 3.8 mmol/L

cNa+ 130 mmol/L

cCa2+ 1.00 mmol/L

cCl- 101 mmol/L

pH 7.271

pCO2 50.5 mmHg / 6.7 kPa

pO2 116.7 mmHg / 15.56 kPa

Sample B ctHb 4.6 mmol/L

cGlu 6.9 mmol/L

cK+ 2.5 mmol/L

cNa+ 137 mmol/L

cCa2+ 0.61 mmol/L

cCl- 113 mmol/L

pH 7.275

pCO2 35.9 mmHg / 4.8 kPa

pO2 129.3 mmHg / 17.2 kPa

19 Prof Asmaa El Reweny, MD

Air bubbles Any air bubbles in the sample must be expelled

as soon as possible after the sample has been drawn;

before mixing the sample with heparin

before cooling of sample.

Even small air bubbles may seriously elevates pO2 value.

An air bubble whose relative volume is 0.5 to 1.0 % of blood is a potential source of a significant error.

Effect of air bubbles - an example

Sample A and B were taken from the same patient immediately after each other

Sample A without air bubbles was analyzed immediately after collection

100 µL air was added to sample B (1 mL). It was stored cold (0-4 °C) for 30 min and mixed for 3 min before sample analysis

Sample A

pO2 288.6 mmHg /38.5 kPa

Sample B

pO2 253.3 mmHg / 33.8 kPa

21 Prof Asmaa El Reweny, MD

Insufficient mixing with heparin

Insufficient mixing can

produce clots.

It is recommended to

mix the blood sample

thoroughly with heparin

Invert the syringe 10

times and roll it between

your palms

22 Prof Asmaa El Reweny, MD

Inadequate mixing - an example

Sample A and B were taken from the same patient

immediately after each other and stored cold for 10 min

Sample A was mixed in a rotator (14 revolutions/min) for

3 min

Sample B was mixed in a rotator (14 revolutions/min) for

1 min

Sample B

ctHb 4.5 mmol/L

Sample A

ctHb 6.2 mmol/L

23 Prof Asmaa El Reweny, MD

Hemolysis Hemolysis may easily occur during blood

sampling.

Hemolysis may occur due to

high filling pressure through narrow entrance

(e.g during too vigorous sample aspiration,

sample transfer to the analyzer, etc.)

vigorous rubbing or squeezing of the skin

during capillary sampling

too vigorous mixing of the sample

cooling down the sample < 0 °C.

24 Prof Asmaa El Reweny, MD

Finally…

The human role in sample collection makes complete elimination of errors associated with laboratory testing unrealistic

However, good practices and compliance with the new strategies for error prevention can lead to a substantial reduction in pre-analytical errors.

25 Prof Asmaa El Reweny, MD

الحمد هلل رب العالمين

Thank You

26 Prof Asmaa El Reweny, MD