Measurement Uncertainty Vs Total Error

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Measurement Uncertainty Vs Total Error

In a recent article, Error Methods Are More Practical, But Uncertainty Methods May Still Be Preferred, James Westgard comments on the latest developments in the debate on the use of analytical total error (TE) and measurement uncertainty (MU), a debate which has been regularly revisited for the last twenty years. This blog aims to briefly explore the benefits of MU and TE and attempt to draw a conclusion on which is most beneficial in the clinical laboratory.

Where do errors and uncertainty come from?

Many things can undermine a measurement. Measurements are never made under perfect conditions and in a laboratory, errors and uncertainties can come from (Good Practice Guide No. 11, 2012):

  • The measuring instrument – instruments can suffer from errors including bias, changes due to ageing, wear, poor readability, and noise.
  • The item being measured – the sample may be unstable.
  • The measurement process – the analyte may be difficult to measure
  • ‘Imported’ uncertainties – calibration of the instrument.
  • User error – skill and judgement of the operator can affect the accuracy of a measurement.
  • Sampling issues – the measurements you make must be properly representative of the process you are trying to assess. I.e. not using fully commutable controls will mean your quality control process is not reflective of a true patient sample.

Random and systematic errors

The effects that give rise to uncertainty in a measurement can be either random or systematic, below are some examples of these in a laboratory.

  • Random – bubbles in reagent, temperature fluctuation, poor operator technique.
  • Systematic – sample handling, reagent change, instrument calibration (bias), inappropriate method.
Total Error (TE)

Total Error (TE) or Total Analytical Error (TAE) represents the overall error in a test result that is attributed to imprecision (%CV) and inaccuracy (%Bias), it is the combination of both random and systematic errors. The concept of error assumes that the difference between the measured result and the ‘true value’, or reference quantity value, can be calculated (Oosterhuis et al., 2017).

TE is calculated using the below formula:

TE = %BIAS + (1.96 * %CV)

Measurement Uncertainty (MU)

Measurement Uncertainty is the margin of uncertainty, or doubt, that exists about the result of any measurement.

There is always margin of doubt associated with any measurement as well as the confidence in that doubt, which states how sure we are that the ‘true value’ is within that margin. Both the significance, or interval, and the confidence level are needed to quantify an uncertainty.

For example, a piece of string may measure 20 cm plus or minus 1 cm with a 95% confidence level, so we are 95% sure that the piece of string is between 19 cm and 21 cm in length (Good Practice Guide No. 11, 2012).

Standards such as ISO 15189 require that laboratories must determine uncertainty for each test. Measurement Uncertainty is specifically mentioned in section 5.5.8.3:

The laboratory shall determine measurement uncertainty for each measurement procedure in the examination phases used to report measured quantity values on patients’ samples. The laboratory shall define the performance requirements for the measurement uncertainty of each measurement procedure and regularly review estimates of measurement uncertainty.”

Uncertainty is calculated using the below formula:


u = √A2+B2          

                                           U = 2 x u               

Where:
A = SD of the Intra-assay precision
B = SD of the Inter-assay precision
u = Standard Uncertainty
U = Uncertainty of Measurement

Error methods, compared with uncertainty methods, offer simpler, more intuitive and practical procedures for calculating measurement uncertainty and conducting quality assurance in laboratory medicine (Oosterhuis et al., 2018).

Conclusion

It is important not to confuse the terms ‘error’ and ‘uncertainty’.

  • Error is the difference between the measured value and the ‘true value’.
  • Uncertainty is a quantification of the doubt about the measurement result.

Whenever possible we try to correct for any known errors: for example, by applying corrections from calibration certificates. But any error whose value we do not know is a source of uncertainty (Good Practice Guide No. 11, 2012).

While Total Error methods are firmly rooted in laboratory medicine, a transition to the Measurement Uncertainty methods has taken place in other fields of metrology. TE methods are commonly intertwined with quality assurance, analytical performance specifications and Six Sigma methods. However, Total Error and Measurement Uncertainty are different but very closely related and can be complementary when evaluating measurement data.

How Randox can help

Whether you prefer Measurement Uncertainty, Total Error, or believe that they should be used together, Randox can help. Our interlaboratory QC data management software, Acusera 24•7, automatically calculates both Total Error and Measurement Uncertainty. This makes it easier for you to meet the requirements of ISO:15189 and other regulatory bodies.

This is an example of the type of report generated by the 247 software. MU is displayed for each test and each lot of control in use therefore eliminating the need for manual calculation and multiple spreadsheets.

Fig. A

Measurement Uncertainty

Fig. B

Total Error vs Measurement Uncertainty

Fig. A and Fig. B above are examples of report generated by the 24•7 software. Fig.A shows how MU is displayed for each test and each lot of control in use therefore eliminating the need for manual calculation and multiple spreadsheets. Fig. B shows TE displayed for each test.

Measurement Uncertainty vs Total Error
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References

Good Practice Guide No. 11. (2012). Retrieved from http://publications.npl.co.uk/npl_web/pdf/mgpg11.pdf

Hill, E. (2017). Improving Laboratory Performance Through Quality Control.

Oosterhuis, W., Bayat, H., Armbruster, D., Coskun, A., Freeman, K., & Kallner, A. et al. (2017). The use of error and uncertainty methods in the medical laboratory. Clinical Chemistry and Laboratory Medicine (CCLM)56(2). http://dx.doi.org/10.1515/cclm-2017-0341

Westgard, J. (2018). Error Methods Are More Practical, But Uncertainty Methods May Still Be Preferred. Clinical Chemistry64(4), 636-638. http://dx.doi.org/10.1373/clinchem.2017.284406


The Importance of Meeting ISO 15189 Requirements

Laboratory accreditation provides formal recognition to competent laboratories, providing a means for customers to identify and select reliable services (CALA, n.d.). Use of accreditation standards by clinical laboratories enables them to drive gains in quality, customer satisfaction, and financial performance. This is essential at a time when laboratory budgets are shrinking.

Some key benefits include:

  • Recognition of testing competence – as mentioned above, customers can recognise the competence of a lab with an internationally recognised standard.
  • Marketing advantage – accreditation can be an effective marketing tool as labs can demonstrate their quality and overall competence.
  • Benchmark for performance – laboratories can determine whether they are performing to the appropriate standards and provides them with a benchmark to maintain that standard.

To maintain the global recognition gained from accreditation, labs are evaluated regularly by an accreditation body to ensure their continued compliance with requirements, and to check that standards are being maintained. (CALA, n.d.).

Impact on healthcare

In a comprehensive study conducted by Rohr et al. (2016) it was found that, while accounting for as little as 2% of total healthcare expenditure, in vitro diagnostics (IVD) account for 66% (two thirds) of clinical decisions. Despite such a small percentage of budget dedicated to it, IVD plays a huge role in patient care so it is vital that there is guidance in place to ensure quality standards are met. Poor performance of tests at any stage of care and treatment can reduce the effectiveness of treatment and deny appropriate care to patients in need (Peter et al., 2010).

ISO 15189

ISO 15189 is an international accreditation standard that specifies the quality management system requirements particular to medical laboratories and exists to encourage interlaboratory standardisation, it is recognised globally.

Meeting ISO Requirements

Scroll through below to learn how ISO 15189 regulates aspects of a clinical laboratory and how Randox can help you meet these suggestions.

The Importance of Meeting ISO 15189 Requirements

At a conference in Belgium in 2016, data, which highlighted the most common areas of non-conformance in laboratories, showed that nonconformities were most prevalent in sections 5.5 and 5.6 of ISO 15189. This data is visualised in fig. A below. Furthermore, a study by Munene et al. (2017) has had similar findings, as visualised in fig. B. The greatest number of nonconformities occur in the sections that are concerned with insufficient assay validation and quality of examination procedures. These studies specifically identified the lack of independent controls, QC not at clinically relevant levels, commutability issues, and a lack of interlaboratory comparison as major issues.

Randox Quality Control products are designed to target these areas, making it easier to conform to ISO 15189 standards.

Fig. A

ISO 15189 requirements - non-conformances

Fig. B

ISO 15189 requirements - non-conformances
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Interlaboratory Data Management

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References

CALA. The Advantages of Being an Accredited Laboratory. Canadian Association for Laboratory Accreditation. Retrieved from http://www.cala.ca/ilac_the_advantages_of_being.pdf

Munene, S., Songok, J., Munene, D., & Carter, J. (2017). Implementing a regional integrated laboratory proficiency testing scheme for peripheral health facilities in East Africa. Biochemia Medica, 110-113. http://dx.doi.org/10.11613/bm.2017.014

Peter, T., Rotz, P., Blair, D., Khine, A., Freeman, R., & Murtagh, M. (2010). Impact of Laboratory Accreditation on Patient Care and the Health System. American Journal Of Clinical Pathology134(4), 550-555. http://dx.doi.org/10.1309/ajcph1skq1hnwghf

Rohr, U., Binder, C., Dieterle, T., Giusti, F., Messina, C., & Toerien, E. et al. (2016). The Value of In Vitro Diagnostic Testing in Medical Practice: A Status Report. PLOS ONE11(3), e0149856. http://dx.doi.org/10.1371/journal.pone.0149856


Benefits of High-Sensitivity Troponin I (hs-TnI)

Benefits of High-Sensitivity Troponin I (hs-TnI)

Chest pain is a common symptom; 20% to 40% of the population will experience chest pain during their lifetime. There are many causes of chest pain, some of which are benign, while others are potentially life threatening. Importantly, in patients with chest pain caused by an acute coronary syndrome (ACS) or angina, there are effective treatments to improve symptoms and prolong life, emphasising the importance of early diagnosis in patients where chest pain may be of cardiac origin (Skinner et al, 2010). Chest pain is one of the most common reasons for emergency admission to hospital and is a heavy burden on health-care resources. A strategy to identify low-risk patients suitable for immediate discharge would have major benefits (Shah et al., 2015).

Case Study - Royal Wolverhampton NHS Trust

In 2012, all patients attending Royal Wolverhampton NHS Trust (RWT) with potential cardiac chest pain were admitted to the acute medical unit where a blood sample was collected, 12 hours post pain onset, for cardiac troponin T testing to aid in the exclusion or confirmation of acute myocardial infarction. A review of the trust’s chest pain pathway, by a consultant acute care physician, was conducted following a need to increase patient discharge rates and reduce hospital admissions.

The introduction of high-sensitivity troponin I (hs-TnI) allowed clinical practitioners in the UK to implement a novel and radically different chest pain pathway. The new pathway uses an admission hs-TnI of <1.9ng/L to discharge patients with suspected acute coronary syndrome (ACS).

The percentage of chest pain patients admitted to the hospital declined from 60.9% to 38.4% and the mean length of stay reduced from 23 hours 2 minutes to 9 hours 36 minutes. (Ford, 2017)

What it means

The adoption of high-sensitivity Troponin I (hsTnI) has allowed RWT to relieve pressure on their emergency department by discharging patients with a hs-TnI level below 1.9ng/L, the limit of detection for the assay.

Randox Liquid Cardiac Control

Accurate test results are vitally important in diagnosis of chest pain and effective quality control is crucial to achieving this.  The Randox Liquid Cardiac Control Level 1 features ultra-low levels of Troponin I, enabling it to challenge the sensitivity of analysers, ensuring confidence in vital clinical decisions.

Features & Benefits

Liquid ready-to-use – enables ultimate ease-of-use, saving time and money. Suitable for both clinical laboratories and point-of-care testing (POCT), where important clinical decisions are made.

100% human serum – ensures complete commutability whereby the control mimics a real human sample, reducing annoying shifts with a change in reagent batch.

Consolidation – comprising an impressive 8 cardiac markers, featuring ultra-low levels of Troponin I meaning a separate high-sensitivity control is not required.

Open vial stability of 20 days at 2°C – 8°C for all analytes – enables less wastage, saving money.

Related Products

RIQAS Liquid Cardiac Programme

Interlaboratory data Management

Benefits of High-Sensitivity Troponin I (hs-TnI)
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References

Ford, C. (2017). Benefits of High Sensitivity Cardiac Troponin I at Admission. Clinical Laboratory Management Association, (July/August 2017), 22-24.

Shah, A., Anand, A., Sandoval, Y., Lee, K., Smith, S., & Adamson, P. et al. (2015). High-sensitivity cardiac troponin I at presentation in patients with suspected acute coronary syndrome: a cohort study. The Lancet386(10012), 2481-2488. http://dx.doi.org/10.1016/s0140-6736(15)00391-8

Skinner, J., Smeeth, L., Kendall, J., Adams, P., & Timmis, A. (2010). NICE guidance. Chest pain of recent onset: assessment and diagnosis of recent onset chest pain or discomfort of suspected cardiac origin. Heart96(12), 974-978. http://dx.doi.org/10.1136/hrt.2009.190066


Get your teeth into a Randox commutable control this Halloween

Get your teeth into a Randox commutable control this Halloween

It is that time of year again – when people dress up, children trick-or-treat and many a scary story is told in households around the world. An age-old tradition celebrated globally by millions of people – it can only be Halloween.

Last year Randox QC brought you the truly scary story about a laboratory who chose not to use a third party control, but eventually “treated their laboratory to a true third party control”. This year, we have another scary story for you about a lab manager in Transylvania, Dr. Acula.

It was a normal, busy day in the lab for Dr. Acula. That was until it was time to change reagent batch, after changing batch of reagent Dr. Acula was shocked to find his QC results had shifted by over 20%.  This left Dr. Acula very frustrated, having to spend precious time troubleshooting and reassigning QC targets. After troubleshooting showed no apparent root cause, Dr. Acula searched the internet for an answer finally stumbling upon an educational guide from Randox Quality Control on commutability and its many benefits to the lab.

Grinning from ear-to-ear with excitement, Dr. Acula began to read the guide in the hope of finding a solution to his problem – and solutions he found. While reading the guide, Dr. Acula came across a quote from ISO 15189:2012.  It read that laboratories “must use quality control materials that react to the examining system in a manner as close as possible to the patient sample”.

Dr. Acula made a decision to look for a commutable control material that met all of his requirements and he didn’t have to search very far. Randox Quality Control were able to supply Dr. Acula and his laboratory with a QC material to meet all his needs – true third party, excellent stability, consistency and consolidation but most importantly of all commutable controls.  The fact all Randox immunoassay and immunology controls are manufactured from 100% human material appealed to Dr. Acula a lot.  After trialing the Randox control material alongside patient samples and comparing results between reagent batches, Dr. Acula was thrilled with the results.

Labs rely heavily on quality control to detect errors in their test system and to ultimately make critical decisions regarding the accuracy and reliability of patient test results, the use of a control that reacts to the test system in the same manner as a patient sample is therefore essential.

At Randox Quality Control we take quality seriously. All our QC products are manufactured to the highest possible standard ensuring controls of unrivalled quality time and time again. Designed to be commutable, the Acusera range will ensure accurate and reliable instrument performance while simultaneously helping laboratories meet ISO 15189:2012 requirements.

Just ask Dr. Acula, who likes our 100% human controls so much he has started to drink them himself!


Don’t Get Tricked This Halloween

Don’t Get Tricked This Halloween – Treat Your Lab to Randox True Third Party Controls Today!

Halloween – a celebration observed by many countries around the world on a yearly basis. Falling on October 31st this holiday is a chance for people to dress up, carve pumpkins, bob for apples, attend costume parties, trick-or-treat and tell scary stories.

It just so happens that we have a scary story for you – and what makes this story even scarier is that it’s a true story!

Our story starts off in a medical laboratory. This laboratory was running QC on their machine as they would do every day. Getting accurate results with no faults or problems arising from their machine, this laboratory was happy with how things were going – until one day when it all went wrong!

Having run their EQA/PT samples, the laboratory found themselves reviewing their report with shock –they noticed a large negative bias. To their horror the perceived ‘accuracy’   they had once achieved was now no longer the case. Right away the laboratory professional’s thoughts turned to the fact that approx. 70% of all clinical decisions are based on laboratory test results, meaning it is essential that the results provided are accurate and reliable in order to prevent potential misdiagnosis or inappropriate treatment. Had they sent incorrect patient results to the clinicians? Had a patient been misdiagnosed? Many thoughts fluttered around in their heads.

The laboratory repeated their QC and found that the results obtained were almost identical to the previous run. The laboratory knew there must be a problem with their QC or their instrument, so they began the troubleshooting process. Nothing. Nada. Zilch. “What was going on?” was the question on the lips of the laboratory professionals.

One of the laboratory professionals then stumbled across a case study that took place in the University of Verona and Academic Hospital of Parma, Italy. The study was related to a field recall of Intact PTH, the reagent was recalled after falsely elevated patient results were discovered.  The alarming thing was that the same elevated performance was not identified by the instrument manufacturer’s control. The study reported that due to this issue there was potential for 40,000 inaccurate patient results from just 18 labs in the Lombardy region of Italy. The study also concluded that the issue could have been prevented if a third-party control, independent from calibrator materials had been used.

This PTH case study got the laboratory thinking that maybe they should source a true third party manufacturer… Having sampled a third party QC, the lab found their results now mirrored that of their EQA and patient samples and as such proceeded to make the switch from first party to third party.

The moral of this story is that first party controls can sometimes “trick” the lab into thinking their performance is acceptable. Quite often target values provided with first party quality controls are in the middle of the analytical range thus masking the issues at the low and high ends of the assay range. Laboratory professionals should “treat” their labs to the best QC material. ISO 15189 highlights that the “use of independent third party control materials should be considered, either instead of, or in addition to, any control materials supplied by the reagent or instrument manufacturer”. So this Halloween don’t randomly choose your QC supplier, treat your laboratory to the best, Randox QC.

All Randox controls are manufactured independently of any instrument or reagent, and designed for use with multiple instruments and methods ensuring, unbiased performance assessment.


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