Preanalytical considerations

“Collection of a blood specimen, as well as its handling and transport, are key factors in the accuracy of clinical laboratory analysis and ultimately in delivering quality patient care... In blood gas and pH analysis incorrect results can often be worse for the patient than no result at all”.

National Committee for Clinical Laboratory Standards: Blood gas preanalytical considerations: specimen collection, calibration and controls. NCCLS Document C27-A. 1993: 13(6).]

The preanalytical phase

The preanalytical phase – prior to inserting the sample into the analyzer – is the largest contributor of bias to blood gas measurement results. Inappropriate sampling devices and improper handling can cause substantial inaccuracies in blood gas analysis.

Following a few straightforward recommendations can reduce preanalytical errors. These recommendations are summarized below and explained in greater detail further down. 

Summary of preanalytical recommendations

• Preferably take the sample at a time reflecting a true picture of the patient’s condition
• Remove air bubbles and mix the sample immediately after collection
• Use dry electrolyte-balanced heparin as anticoagulant to avoid dilution errors and bias of electrolyte values
• Minimize storage to avoid the continued effect of metabolism, oxygen diffusion, and potassium leakage from the cell that bias the results
• Mix the sample thoroughly and remove the first drops of blood before analysis to avoid analyses of inhomogeneous sample
• Avoid hemolysis by using a gentle sample collection and mixing technique

Before sampling

A blood sample reflects patient status at the time when the sample was taken. To get a true picture of the patient’s condition, samples should be taken during a period that is reflective of the patient’s overall condition. Remember to record the time when the sample is taken. The blood gas test should preferably be taken when the patient is stable.

The patient's overall respiratory and circulatory condition should be observed at the moment of sampling in order to interpret the blood gas values in relation to the other diagnosis parameters.

The blood sampler should contain sufficient heparin to prevent coagulation. Inadequate amounts of heparin can cause blood clots, which ultimately may block the analyzer or lead to biased results.

Use preheparinized blood samplers with dry heparin. Liquid heparin dilutes the sample and alters the true value of the sample, often by more than 10 %.

When measuring electrolytes, use electrolyte-balanced heparin to prevent biases. Non-electrolyte-balanced heparin will often bias the sample, as heparin binds with cations, e.g., calcium or potassium.

Immediately after sampling

If air bubbles are present in the syringe, remove them immediately:

1.	Cover the tip of the syringe with a piece of gauze
2.	Tap the syringe while holding it vertically
3.	Expel the air bubbles

Once the air bubbles have been expelled, the sample should be closed with a tip cap and mixed thoroughly to dissolve the heparin. Failure to do so may lead to the formation of micro clots, which in turn can bias results, interfere with measurements, and lead to analyzer downtime.

A patient ID label should be placed on the sampler barrel together with other information, such as sampling time, sampling site and type of sample, patient temperature, ventilator settings, etc.

Patient temperature and FO₂(I) values should be recorded, as they are necessary for the interpretation of the blood gas analysis. FO₂(I) is also needed for the correct calculation of FShunt. By entering the patient temperature into the blood gas instrument when analyzing the sample, the analyzer will display temperature-corrected results.

Storage and transport

Plastic syringes
Storage should be avoided whenever possible or, at least, kept to a minimum. If it is not possible to analyze the sample immediately, store it at room temperature and analyze it within 30 minutes after collection.Samples with expected high pO₂ values or for special studies like shunt studies should be analyzed immediately or within 5 minutes.

Glass syringes
Storage should be avoided whenever possible or, at least, kept to a minimum. If it is not possible to analyze the sample immediately, store it at room temperature and analyze it within 30 minutes after collection. Alternatively, store the sample in ice water (0-4 °C). The storage time should not exceed 1 hour. Samples with expected high pO₂ values or for special studies like shunt studies should be analyzed immediately or within 5 minutes.

Just before analysis

The portion of the sample transferred to the analyzer must be homogeneous and representative of the whole sample. If not, significant errors can occur, particularly on the hemoglobin parameters. Mix the sample thoroughly by repeatedly inverting it and rolling it horizontally.

A sample that has been stored for 30 minutes may have settled completely, thus requiring thorough mixing. The first few drops of blood from the tip of the syringe are often coagulated and not representative of the whole sample. Consequently, a few drops of blood should always be expelled, e.g., on a piece of gauze, before inserting the sample into the analyzer.

Evaluating test results

Before reporting results, evaluate whether the results may have been biased, particularly if they differ from the general assessment of the patient’s condition. If there is any sign of bias, report it along with the results and take it into account when making a clinical decision.

Arterial sample types

Arterial samples
Arterial samples can be collected either by arterial puncture or by aspiration from an indwelling arterial catheter. Both methods have advantages and disadvantages.

Arterial punctures
Advantages	Disadvantages
Less risk of bias than arterial-line and capillaries, if performed correctly Can be carried out in an emergency situation No catheter needed Requires less blood volume than catheter sampling	Painful to the patient, hyperventilation can potentially change blood gas values It can be difficult to locate arteries Risk of complications for the patient, not always advisable to perform arterial puncture Operator safety - risk of needle stick accidents Requires trained/authorized personnel

Arterial catheter line
Advantages	Disadvantages
Easy to obtain samples, because of indwelling line Not painful to the patient Elimination of risk associated with multiple punctures	Risk of air contamination to the sample from catheter connections, etc. Risk of sample dilution errors, if catheter is flushed insufficiently Risk of infection with invasive catheter Clotting may lead to thromboses or emboli Risk of anemia due to removal of too much blood (typically 5-6 mL per sample including waste) Locally diminished or blocked blood flow may lead to necrosis

Capillary samples
Capillary samples are often used to evaluate arterial oxygen status. However, use this method with caution:

• Capillary samples depend on the peripheral circulation and may therefore differ from arterial values
• The method is difficult to master, and should therefore only be performed by skilled personnel
• Air contamination of the sample is frequent and can cause significant changes in all respiratory parameters
• Hemolysis can cause changes in the electrolyte status
• Measures of oxygen status obtained from a capillary sample must always be interpreted with caution.

Venous samples
Peripheral venous samples are not recommended for the evaluation of the oxygen status, as they provide little or no information on the general status of the patient. Samples obtained from central venous catheters can be used to evaluate mixed venous oxygen status.

Misleading results can, however, be obtained if the sample is collected primarily from either the superior or the inferior vascular beds, or if cardiac left-to-right shunt on the arterial level is present.

Oxygen status in mixed venous blood collected from a catheter with its tip placed in the pulmonary artery is a useful tool to evaluate respiratory, metabolic, and circulatory status of the patient.

A low mixed venous oxygen content is a sign of impaired oxygen supply due either to low arterial oxygen availability or circulatory insufficiency with increased oxygen extraction. As the ctO₂ may be low, air contamination of a mixed venous sample may cause a relatively higher bias in oxygen parameters than similar air contamination of an arterial sample.