Cece wrote:I said it wrong, didn't I. Here's Siskin's research on the diagnostic utility of ultrasound in CCSVI:
http://www.sirmeeting.org/index.cfm?do= ... s&abs=2088
Conclusions: Findings on a Zamboni-protocol US are not associated with findings on contrast venography in light of the high false negative rate; 99% of the patients with a negative US had a significant stenosis and/or flow abnormality treated with angioplasty. A prospective study is needed to define the role of US and other modalities in the non-invasive diagnosis of CCSVI.
So the 1% would, 99% of the time, be a false negative.
I remember you had a false positive! Showed CCSVI on doppler but no CCSVI during the procedure. An unusual turn of events.
I dont interpret this data the way you describe it now either.
A false negative rate is the rate at which a negative result is false. Dr Almond tells us that of the 86 patients with negative ultrasound exam only 20 were negative on venography. The remaining 66 patients with a negative ultrasound, had a positive venogram.
So the false negative rate is 66/86 or 77%
Similarly of the ultrasounds that were positive (224) only 155 had a positive venogram. So the false positive exams are 69 ultrasounds. The false positive rate would therefore be 69/224. That would be 31%
So these are false positive and false negative rates of ultrasound on
this study.
But this is based upon venography being the Gold standard of comparison. If you use autopsy as the gold standard or venography plus IVUS as the gold standard the results would be very different.
Moreover, the gold standard criteria of venography that were used were
1. flow disturbance
2. greater than 50% stenosis.
the signs of flow disturbance, such as reflux, stasis and collateral visualization, is very subjective and depends upon rate and pressure of contrast injection and subjective observation
It is highly debatable that stenoses less than 50% are not significant . the 50% criteria was declared without much proof for significance in arterial disease where higher pressures would enable flow through lesser stenoses which perhaps is not the case in venous obstructions.
There are fluid dynamicists who would argue for a much lower percentage as signficant.
So the conclusions themselves are subjective and i think that the interpretation of the ultrasounds and the interpretation of the venograms may explain the high false positive and high false negative rates.
Ultrasound is a screening test. the Goals of a screening test are to safely differentiate someone with a disease or disorder from someone who does not. This is economically and medically essential because we do not want to perform even minimally invasive tests on healthy patients and we dont want to miss an opportunity to treat some who has a disease.
We use the concepts of sensitivity and specificity to evaluate these concepts. Sensitivity is the ability to detect maximum numbers of patients with ccsvi. sensitivity is the ability to correctly exclude healthy patients from the diagnosis.
We balance sensitivity and specificity to determine whether a test is a good one by using good judgment about risks, about complications, about consequences of missing the disease. So we accept fairly high false positive rates in breast cancer screening because we want to keep the false negative rate as low as possible. We accept that we will do biopsies of lumps that are not cancer (false positive) because to miss cancer is a potentially lethal consequence.
So lets look at the specificity and sensitivity of ultrasound for ccsvi from this study
sensitivity tells us what percentage of patients who have the disease will be detected.
Sensitivity is the number of true positive / true positive + false negative, in other words what percentage of people with ccsvi will be detected by this test
dr almond said there were 155 true positive ultrasound and 66 false negative ultrasound
so the sensitivity is 155/155+66 or 155/221 or 70% sensitivity.
Specificity tells us what percentage of patients who do not have the disease are truly negative
specificity is the number of true negatives / true negatives+false positives
Dr almond said that 20 patients had a true negative ultrasound and 69 patients who had a false positive ultrasound
so the specificity of ultrasound using two parameters of a positive venogram is
20/20+69 or 20/89 or 22%
Sensitivity of 70% and specificity of 22% leave much to be desired. 30% of patients with positive US results will undergo unnecessary venography and 76% of patients who have a negative ultrasound will not get a venogram that can diagnose and treat their ccsvi
According to the Albany data's sensitivity and specificity results, ultrasound would not a very good screening test. it misses too much disease and adds too much cost by doing venography in too many patients without disease.
is that understandable? The conclusions are debatable but not based upon the data of their paper.
.
DrS
