10a. Index test
What to write
Index test, in sufficient detail to allow replication.
Explanation
(NB the explanation and example for items 10a and 10b are the same.)
Differences in the execution of the index test or reference standard are a potential source of variation in diagnostic accuracy.1,2 Authors should therefore describe the methods for executing the index test and reference standard, in sufficient detail to allow other researchers to replicate the study, and to allow readers to assess (1) the feasibility of using the index test in their own setting, (2) the adequacy of the reference standard and (3) the applicability of the results to their clinical question.
The description should cover key elements of the test protocol, including details of:
the preanalytical phase, for example, patient preparation such as fasting/feeding status prior to blood sampling, the handling of the sample prior to testing and its limitations (such as sample instability), or the anatomic site of measurement;
the analytical phase, including materials and instruments and analytical procedures;
the postanalytical phase, such as calculations of risk scores using analytical results and other variables.
Between-study variability in measures of test accuracy due to differences in test protocol has been documented for a number of tests, including the use of hyperventilation prior to exercise ECG and the use of tomography for exercise thallium scintigraphy.2,3
The number, training and expertise of the persons executing and reading the index test and the reference standard may also be critical. Many studies have shown between-reader variability, especially in the field of imaging.4,5 The quality of reading has also been shown to be affected in cytology and microbiology by professional background, expertise and prior training to improve interpretation and to reduce interobserver variation.6 Information about the amount of training of the persons in the study who read the index test can help readers to judge whether similar results are achievable in their own settings.
In some cases, a study depends on multiple reference standards. Patients with lesions on an imaging test under evaluation may, for example, undergo biopsy with a final diagnosis based on histology, whereas patients without lesions on the index test undergo clinical follow-up as reference standard. This could be a potential source of bias, so authors should specify which patient groups received which reference standard.7,8
More specific guidance for specialised fields of testing, or certain types of tests, will be developed in future STARD extensions. Whenever available, these extensions will be made available on the STARD pages at the EQUATOR (Enhancing the QUAlity and Transparency Of health Research) website (http://www.equator-network.org/).
In the example, the authors described how blood samples were collected and processed in the laboratory. They also report analytical performance characteristics of the index test device, as obtained in previous studies.
Example
‘An intravenous line was inserted in an antecubital vein and blood samples were collected into serum tubes before (baseline), immediately after, and 1.5 and 4.5 h after stress testing. Blood samples were put on ice, processed within 1 h of collection, and later stored at −80°C before analysis. The samples had been through 1 thaw–freeze cycle before cardiac troponin I (cTnI) analysis. We measured cTnI by a prototype hs assay (ARCHITECT STAT high-sensitivity troponin, Abbott Diagnostics) with the capture antibody detecting epitopes 24–40 and the detection antibody epitopes 41–49 of cTnI. The limit of detection (LoD) for the high sensitivity (hs) cTnI assay was recently reported by other groups to be 1.2 ng/L, the 99th percentile 16 ng/L, and the assay 10% coefficient of variation (CV) 3.0 ng/L. […] Samples with concentrations below the range of the assays were assigned values of 1.2 […] for cTnI. […]’.9
Training
The UK EQUATOR Centre runs training on how to write using reporting guidelines.
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