screening tests 101

Screening tests 101

Sagefemme is confused about the difference between screening tests and

diagnostic tests. This is really unconscionable. How can a provider

accurately counsel patients about prenatal testing if she doesn't even

understand the basics? Obviously, she can't. Like most direct entry

midwives, she hasn't a clue what she is talking about, and therefore

spreads misinformation to others.

Her latest post, Prenatal testing is an object lesson on why direct

entry midwives should be required to have far more education before

anyone should consider licensing them.

The issue I have with standard prenatal testing is the myth that

somehow each test that is done will ensure a healthy baby or point

out any issues with a baby with special needs. The myth that each

procedure (the biggest offenders - the Alphafetoprotein screen and

ultrasound - which are not diagnostic, just screens) offers an

accurate diagnosis of any issue - and if it's not apparent with

these tests then your baby must be ok.

There is so much wrong with these statements that it is difficult to

know where to begin. Let's take it one sentence at a time:

"The issue I have with standard prenatal testing is the myth that

somehow each test that is done will ensure a healthy baby or point out

any issues with a baby with special needs."

Standard prenatal testing is made up predominantly of screening tests.

Screening tests, BY DEFINITION, cannot ensure a healthy baby or point

out any special needs. Screening tests do one and ONLY one thing; they

identify people AT RISK for specific conditions. Screening tests, BY

DEFINITION, will identify a much larger group of people than will

actually have the condition at issue. Those people will then be tested

with the diagnostic test which is more complicated, more inconvenient

and more expensive than the screening test.

So, for example, if we want to know which babies have Down's syndrome,

we could perform amniocentesis on all 4 million pregnant women each

year OR we can screen women with the AFP blood test, identify the few

tens of thousands that are at risk and offer those women

amniocentesis. Which makes more sense: 4 million amnios or 4 million

AFP blood tests followed by 40 thousand amnios? Obviously, performing

the blood test and limiting the numbers of amniocentesis makes more

sense from the point of view of safety, convenience and expense.

"The myth that each procedure (the biggest offenders - the

Alphafetoprotein screen and ultrasound - which are not diagnostic,

just screens) offers an accurate diagnosis of any issue - and if it's

not apparent with these tests then your baby must be ok."

Wrong again. By definition, any test which is not a diagnostic test

CANNOT provide a diagnosis. This appears to be news to Sagefemme, but

it is statistics 101. EVERY screening test has a false positive rate

and identifies women AT RISK who do not actually have the diagnosis.

Every screening test has a false negative rate, which means that a

certain percentage of people identified as not at risk actually have

the disease or condition. If you read the consent forms for AFP tests

for example, they specifically state that there is a chance that the

test will fail to identify a certain percentage of babies with Down's

syndrome. That is the nature of screening tests. If you want to be

sure that your baby does not have Down's syndrome, you MUST have an

amniocentesis (the diagnostic test).

"Current evidence shows us that the Alphafetoprotein test, routine

ultrasound and the glucose tolerance test all have rather large errors

in assisting with an accurate diagnosis."

Yes, they were DESIGNED to work in exactly this way. They have a known

(and deliberately high) false positive rate and a known false negative

rate. They cannot make a diagnosis because they are not diagnostic

tests.

"If we're really looking at empowering women with knowledge the

accuracy rate, along with what the test is screening for and what the

path is if an abnormal result is found, should be discussed prior to

the test."

Yes, Sagefemme, but how is a direct entry midwife going to do that

when she herself doesn't understand the test?

"My issue isn't with the testing alone, but the inability of the

medical model to offer full disclosure about testing and it's

accuracy."

No, it is DEMs who can't offer full disclosure about testing and its

accuracy because they don't understand it. This is what I mean when I

say that DEMs are grossly undereducated.

"For what it's worth, each of my clients receive full informed choice

about Chorionic Villus Sampling, Amniocentesis, Ultrasound, AFP

Screening, the Glucose Tolerance Test, Group Beta Strep Testing"

No, Sagefemme, they couldn't, because you clearly don't understand

prenatal testing, the difference between screening tests and

diagnostic tests, false positive rates, false negative rates and other

basic concepts in testing. If you don't understand it, you can't offer

accurate counseling. That's a very serious problem.

Sagefemme concludes:

Informed choice and full disclosure should be the standard in

maternity care. We also owe it to women to have an understanding

about the impact of prenatal testing on the feelings towards their

pregnancy, towards their baby and the choices made along the way.

I agree, that should be the standard. Sagefemme, you are ethically and

legally obligated to educate YOURSELF on the basics of prenatal

testing so you can offer accurate information. Until now, you have

been offering misinformation.

posted by Amy Tuteur, MD @ 10:47 PM