EXTERNAL QUALITY ASSESSMENT


when oxygenating in room air; with values < 8.0 kPa (< 60 mmHg) indicating respiratory failure.


Although blood gas analysers have been developed and refined over recent decades, the technology used to measure pO2


remains largely similar


to that used in the earliest instruments. The Clark oxygen sensor, introduced in 1958, relies on reduction of oxygen to hydroxy ions to generate an electrical current, via a silver/silver chloride and platinum electrode system.3


As oxygen


present in blood diffuses across an oxygen-permeable membrane and into the electrode, the current changes proportionally to the concentration of oxygen in the sample, providing an assessment of pO2


in blood.


Fig 1. The oxyhaemoglobin dissociation curve models the relationship between pO2 oxygen saturation. At normal arterial pO2


, small changes in pO2


haemoglobin and hence delivery to the tissues, but as pO2 considerably impaired, with pO2


by non-laboratory personnel. The clinical impact of POCT blood gas analysers is significant: reducing time to treatment in acute medicine, leading to improvement in patient outcomes and reducing workload for central laboratories. However, maintaining competency of users, managing cost of consumables, and standardisation of different devices remains a challenge for pathology services.


The first clinical question asked as part of interpretation of ABG is whether the patient is hypoxic, requiring supplemental oxygen, as this is the most immediate threat to life.2


The


oxygenation status of the patient is assessed by pO2


. This parameter


measures the amount of oxygen dissolved in the patient’s blood, which is not bound to haemoglobin. Oxygen is poorly soluble in blood, and > 98 % of the total oxygen content of the blood is bound to haemoglobin – forming oxyhaemoglobin. There are


<8 kPa indicating respiratory failure.


several factors determining whether oxygen is bound to haemoglobin, including pO2


. When pO2 is high,


such as in the alveolar blood vessels, haemoglobin has a much higher affinity for oxygen and forms oxyhaemoglobin. When pO2


is reduced, such as in


is lower, such as in peripheral tissues, haemoglobin affinity for oxygen decreases, and oxygen is released into the blood and diffuses into the tissues. If pO2


asthma, pneumonia or other lung diseases, less oxygen can be carried by haemoglobin, and less oxygen is delivered to the tissues. According to the oxygen dissociation curve which


models the relationship between pO2 and haemoglobin oxygen saturation, small changes in pO2


on saturation of haemoglobin and hence delivery to the tissues, but as pO2


typically 10-13 kPa (75-100 mmHg) have little effect


falls below 10 kPa, oxygen delivery becomes considerably impaired (Fig 1). In a healthy patient, arterial pO2


is and haemoglobin have little effect on saturation of falls below 10 kPa, oxygen delivery becomes


Modern blood gas analysers feature improvements over the original design, such as miniaturisation of components, improved membrane composition, allowing for better accuracy and response time, automated temperature controls and better calibration processes. Some manufacturers have moved away from the Clark electrode and have incorporated optical sensors for measuring pO2


into instruments.4 Optical


sensors comprise a luminescent dye embedded in a matrix, which emits light with intensity inversely proportional to the concentration of oxygen in the sample. Although their use is currently limited in clinical practice, these devices require a reduced level of calibration and maintenance and have longer lifespan than the Clark electrode; and may be particularly applicable to continuous monitoring devices.


External quality assessment As with all laboratory analysers and point-of-care instruments, external quality assessment (EQA) is essential to ensure that blood gas analysers provide accurate results that are appropriate for patient care. Since April 1995, Weqas has distributed three EQA samples monthly for blood gas analysers at both laboratory and POCT testing sites, covering H+ , electrolytes, glucose and


, pCO2 , pO2


The clinical impact of POCT blood gas analysers is significant. However, maintaining competency of users, managing cost of consumables, and standardisation of different devices remains a challenge for pathology services


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lactate; and since 2008, urea and creatinine. Tonometry with predetermined levels of oxygen and carbon dioxide balanced with nitrogen and different salt concentrations provide six distinct levels for each parameter, simulating clinically significant ranges of acid-base and electrolyte balance, respiratory function, renal function, glucose, and lactate concentration. The standard aqueous material includes the addition of protein which minimises the risk of preanalytical sampling error through exposure to room air, by forming a protein foam layer. The


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