LABORATORIO DE URGENCIAS

1. Errors in measuring blood gases in the intensive care unit: Effect of delay in estimation
A. Woolley; K. Hickling - J. of Critical Care,1998, vol 18; Issue 1;: 31-37

Abstract
Arterial blood gas measurement is subject to a number of potential sources of error. We investigated some of these in the intensive care unit (ICU). We audited samples for adequate volume and the presence of air and found that all samples were of adequate volume, but 40% contained bubbles or froth. We compared pulse oximeter estimations of oxygen saturation (SpO₂) with laboratory estimates (SO₂) from arterial blood samples, and found that there was less than a 5% chance of a difference of 5% or more. We audited the delay between sampling and processing and looked for errors arising as a result. We found that 4% of samples waited longer than 30 minutes to be analyzed in the laboratory, but that there was no correlation between delay and error in partial pressure of oxygen (PO₂), carbon dioxide (PCO₂), or SO₂. We performed a bench study to document the changes in PO₂ and PCO₂ over time with samples stored at room temperature and on ice. We found that samples in 1.5-mL PICO 70 syringes (Radiometer Medical A/S, Bronshoj, Denmark) were stable for PO₂ and SO₂ for up to 30 minutes either at room temperature or kept in iced water, and that changes after 60 minutes were small and unlikely to be clinically significant. PCO₂ showed a statistically significant increase after 20 minutes at room temperature, but the changes were not clinically significant.

2. Instrumental variability of respiratory blood gases  Among different blood gas analysers in different laboratories
M.J. Kampelmacher, R.G. van Kesteren, E.K.A. Winckers - Eur Respir J 1997; 10: 1341-1344

Abstract:
The aim of this study was to test the hypothesis that differences in oxygen tension (PO2) and carbon dioxide tension (PCO2) values from measurements performed on different blood gas analysers in  different laboratories are clinica insignificant.
Samples of fresh whole human tonometered blood (PO2 8.1 kPa (60.8  mmHg) PCO2 5.3 kPa (39.9 mmHg)) were placed in airtight glass syringesand transported in ice-water slush. Blood gas analysis was performed within 3.5 h by 17 analysers (10 different models) in 10 hospitals on on day.
The mean of the differences between the measured and target values was - 0.01± 0.19 and 0.21±0.13 kPa (-0.06±1.45 and 1.55±1.01 mmHg) for PO2 and PCO2, respectively.
The mean of the differences between two samples on one analyser was 0.06±0.06 and 0.04±0.03 kPa (0.47±0.48 and 0.29±0.24 mmHg) respectively.
For PO2 and PCO2 the interinstrument standard deviations (sb) were 0.18  and 0.13 kPa (1.38 and 0.99 mmHg), respectively, whereas the intra instrument standard deviations (s) were 0.06 and 0.03 kPa (0.47 and 0.26 mmHg), 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.
A. Immediately after sampling
If air bubbles are present in the syringe, remove them immediately.
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.
B. 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.
C. 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.
D. Types Sample 
Arterial samples
Can be collected either by arterial puncture or by aspiration from an indwelling arterial catheter. Both methods have advantages and disadvantages.

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