Mohamed Mahmoud and Oliver Pratt highlight the most common clinical measurement errors encountered in daily practice and advise as to how they may be avoided
Abstract
The recording of accurate clinical data and physiological parameters is a key aspect of modern anaesthetic and critical care practice. It is a tool which help clinicians in diseases diagnosis, identification of abnormalities, and to guide clinical interventions. Such a tool, if used appropriately, will prevent any potential patient harm or unexpected event. Errors in clinical measurement can lead to either incorrect or missed diagnoses or even failed interventions. A clinician must therefore be aware of the most common errors which affect the accuracy of recording clinical data. Such error should be prevented and the source should be identified and corrected. Errors can be multifactorial in their origin; causes are generally classified as device, operator, or patient related. This article will highlight the most common errors encountered in our daily practice and advise as to how they may be avoided.
Introduction
Measurements of clinical parameters are used to assist medical professionals to identify normal patterns as well as any abnormalities or deviation from the standard. The reliance on such measurements has grown rapidly in the last few years. With the aid of clinical measurement, clinicians can be more certain that their diagnoses, based on objective data, are more likely to be correct.
In 2015, the Association of Anaesthetists of Great Britain & Ireland (AAGBI) published its most recent minimum monitoring standards for patients undergoing general anaesthesia. These minimum monitoring standards include use of a pulse oximeter, non-invasive blood pressure monitor, Electrocardiograph (ECG) and measurement of inspired and expired oxygen, carbon dioxide, nitrous oxide and volatile anaesthetic concentrations (if used). Ventilator parameters such as airway pressures, minute volume and respiratory rate should be recorded. A peripheral nerve stimulator should also be used if neuromuscular blocking drugs are administered. According to the GMC, failure to adhere to such guidance is considered malpractice. National Patient Safety Agency has published many alerts in relation to inappropriate use of monitoring system. For example, in 2009 NPSA highlighted that patients may be harmed if a wrong infusion containing dextrose is attached to keep the arterial line open. This might cause a false high blood sugar reading which, if treated, can cause sever hypoglycaemia and even cardiac arrest.
Components of a typical measurement system
The basic function of any monitoring system is to collect data from a patient or subject and process it to produce a meaningful and reproducible display. Figure 1 shows a schematic representation of a basic clinical measurement system. The first stage involves collection of specific signals from the patient – for example in an ECG this would be electrical impulses caused by cardiac action potentials. The
collected signals are amplified, filtered, and processed by a microcomputer in order to produce a readable display.
Clinical measurement systems must produce data that is both accurate, and precise. These concepts are defined as follows:
Accuracy is defined as the degree of correctness of the measurement when compared to the true or an absolute value. Calibration is an essential step to ensure an accurate reading. It is used to test and optimally adjust measuring instruments. Most devices are calibrated during the manufacturing process, however certain devices require additional calibration before or during usage. One-point calibration is required for a liner relationship. This involves measuring a system output against a known real value, e.g. zeroing of the invasive arterial blood pressure system. Multiple-point calibration involves using three or more know values or concentration, e.g. blood gas machine calibration using different pH solutions.
Precision relates to the reproducibility of the repeated measurement – a monitor which is precise, will give the same reading repeatedly when measuring the same signal. Precision therefore describes the “scatteredness” of data recordings.
We can illustrate the concepts of accuracy and precision by considering a single pin prick test to assess blood Haemoglobin concentration. The test will be will be accurate if the measured readings reflect the true haemoglobin level in blood. The test will be precise if it gives the same result each time a measurement is obtained.
Input-output relationship
Monitoring systems produce an output signal, in response to an input. In an ideal monitor, the magnitude of the output signal is related to the magnitude of the input in a constant, linear fashion.
Filters are incorporated into measurement systems to prevent artefacts or unnecessary