February 24, 2018
A few blog entries ago, I described a case when calculating the SD did not provide an estimate of random error because the observations contained drift.
Any time that data analysis is used to estimate a parameter, there are usually a set of assumptions that must be checked to ensure that the parameter estimate will be valid. In the case of estimating random error from a set of observations from the same sample, an assumption is that the errors are IIDN, which means that the observations are independently and identically distributed in a normal distribution with mean zero and variance sigma squared. This can be checked visually by examining a plot of the observations vs. time, the distribution of the residuals, the residuals vs. time, or any other plot that makes sense.
The model is: Yi = ηi + εi and the residuals are simply YiPredicted – Yi
February 18, 2018
To recall, total analytical error was proposed by Westgard in 1974. It made a lot of sense to me and I proposed to CLSI that a total analytical error standard should be written. This proposal was approved and I formed a subcommittee which I chaired and in 2003, the CLSI standard EP21-A, which is about total analytical error was published.
When it was time to revise the standard – all standards are considered for revision – I realized that the standard had some flaws. Although the original Westgard article was specific to total analytical error, it seemed that to a clinician, any error that contributed to the final result was important regardless of its source. And for me, who often worked in blood gas evaluations, user error was an important contribution to total error.
Hence, I suggested the revision to be about total error, not total analytical error and EP21-A2 drafts had total error in the title. There were some people within the subcommittee and particularly one or two people not on the subcommittee but in CLSI management, who hated the idea, threw me off my own subcommittee and ultimately out of CLSI.
But recently (in 2018) a total error task force published an article which contained the statement, to which I have previously referred:
“Lately, efforts have been made to expand the TAE concept to the evaluation of results of patient samples, including all phases of the total testing process.” (I put in the bolding).
Hence, I’m hoping that the next revision, EP21-A3 will be about total error, not total analytical error.
February 16, 2018
Here’s an article about flash glucose monitoring, a way for diabetic patients to avoid finger sticks and glucose monitors. Now I can understand why other glucose meter companies are trying to get out of the business. This product sounds like a game changer.
February 14, 2018
The 11th International Conference on Advanced Technologies and Treatments for Diabetes (ATTD) is underway in Vienna, Austria. The abstracts from the conference are available here. Here’s an interesting observation: I searched for the term MARD and it was found 48 times whereas the term error grid was found only 10 times. I published a paper describing problems with the MARD statistic and offered alternatives.
February 5, 2018
There is a recent article which says that measurement uncertainty should contain a term for biological variation. The rationale is that diagnostic uncertainty is caused in part by biological variation. My concerns are with how biological variation is turned into goals.
On the Westgard web site, there are some formulas on how to convert biological variation into goals and on another page, there is a list of analytes with biological variation entries and total error goals.
Here are my concerns:
- There are three basic uses of diagnostic tests: screening, diagnosis, and monitoring. It is not clear to me what the goals refer to.
- Monitoring is an important use of diagnostic tests. It makes no sense to construct a total error goal for monitoring that takes between patient biological variation into account. The PSA total error goal is listed at 33.7%. Example: For a patient tested every 3 months after undergoing radiation therapy, a total error goal of 33.7% is too big. Thus, for values of 1.03, 0.94, 1.02, and 1.33, the last value is within goals but in reality would be cause for alarm.
- The web site listing goals has only one goal per assay. Yet, goals often depend on the analyte value, especially for monitoring. For example the glucose goal is listed at 6.96%. But if one examples a Parkes glucose meter error grid, at 200 mg/dL, the error goal to separate harm from no harm is 25%. Hence, the biological goal is too small.
- The formulas on the web site are hard to believe. For example, I < 0.5 * within person biological variation. Why 0.5, and why is it the same for all analytes?
- Biological variation can be thought to have two sources of variation – explained and unexplained – much like in a previous entry where the measured imprecision could be not just random error, but inflated with biases. Thus, PSA could rise due to asymptomatic prostatitis (a condition that by definition that has no symptoms and could be part of a “healthy” cohort). Have explained sources of variation been excluded from the databases? And there can be causes of explained variation other than diseases. For example, exercise can cause PSA to rise in an otherwise healthy person.
- Biological variation makes no sense for a bunch of analytes. For example, blood lead measures exposure to lead. Without lead in the environment, the blood lead would be zero. Similar arguments apply to drugs of abuse and infectious diseases.
- The goals are based on 95% limits from a normal distribution. This leaves up to 5% of results as unspecified. Putting things another way, up to 5% of results could cause serious problems for an assay that meets goals.