Monday, October 5, 2009

Screening Tests---Part I

This blog is the first of a staggered series in which screening tests will be discussed and analyzed. By a screening test, I mean a test done as part of a routine physical, perhaps suggested by the medical history. I define a useful screening test as one which detects a disease at such a time that medical treatment can cure the disease or prolong life.
Since one problem with screening tests is the occurrence of false positives, then either the screening test must be "very" sensitive (few false positives) or else the disease being tested for must have a "high" incidence. The definition of "very" and "high" depends on the disease, the doctor, the patient, the community, and the circumstances. We also would like the sensitive screening test to be able to be supplemented with a more precise test of high specificity (few false negatives). At the present,for instance,the Western Blot is the gold standard for the second test with both Lyme disease, and HIV/AIDS. (I should note parenthetically that the Blood Banks prefer tests of low sensitivity, since they do not want to risk transfusing infected blood.)
There are four reasons for doing screening tests (and again, I want to emphasize that I am NOT talking about diagnostic tests, where a disease is suspected).The reasons are:
a) To benefit the patient (obvious)
b) To benefit the patient's family (but then the patient must give informed consent, since the test will not benefit him directly)
c) To benefit society (again, the patient's consent is required)
d) The patient or a family member suggests it. In this case, it is vital to remember that NO DOCTOR EVER GOT SUED FOR DOING A TEST, but only for not doing one. It does no good to tell a jury that the odds of Leprosy were 1:10,000. If the patient asks for the nine-banded armadillo test, and you refuse, and the patient comes down with leprosy, then the jury will just see that the patient asked for a test, you refused, and he suffered preventable injury as a result.
I am not going to discuss the screening tests to be done on a female who plans to become pregnant, or who is pregnant, or a patient about to be immunosuppressed by steroids,or anti-cancer drugs, or organ transplant treatment, or asplenic patients, etc.
It goes without saying that before any test is done on a patient, he/she should know which tests you are ordering---kidney tests, liver tests, thyroid tests, etc.

Blood pressure---we know that treating high blood pressure (especially systolic) will help prevent strokes. Oddly enough, although there is an entity called hypertensive heart disease, there is not good data to show that lowering blood pressure prevents this.

CBC---detecting anemia leads to a search for the cause and treatment of both the anemia and the underlying cause. Detecting a persistently elevated or low white blood count or platelet count will generally lead to a bone marrow biopsy, which may yield a treatable disease on analysis.

BUN/Creat---elevated kidney tests leads to treatment of early kidney failure.

LFT---elevated liver tests and/or GGTP leads to testing for hepatitis, fatty infiltration of the liver, alcoholism, etc.

Uric acid---elevated in chronic alcoholism, and a warning of risk for gout.

PPD---for certain patients, a skin test for tuberculosis is appropriate.

VDRL---ditto for a screen for syphilis. Caveat: both syphilis and Lyme disease are caused by a spirochete, and Lyme disease is a common cause of a false positive syphilis test. The reflex test for syphilis is FTA-ABS, which turns positive when a patient has syphilis, and almost never reverts to negative. Second caveat: when doing a spinal tap for neurosyphilis, only do the VDRL; nobody knows what to do with a positive CSF FTA.

Stool for blood--- screens for CA of colon, but the most common cause of a positive stool for blood lies in esophageal or gastric irritation/ulceration. Over the age of 65, the cause for the positive stool is not found in 10% of the patients, even when angiodysplasia of the cecum is carefully looked for.

TFT---thyroid abnormality is very common in women, especially after pregnancy, and seems to follow in the female line.

25-OH Vitamin D---low values can contribute to osteoporosis, so females with osteoporosis (and probably those approaching menopause) should be tested.

PSA---as of now, there is no good evidence that treating prostate cancer saves lives, so this test is of doubtful use. Different medical groups recommend for (CA society, urologists) or against (Canadian Health Task Force, USPHS) it.

Fe/TIBC/ferritin---I test all menstruating females and vegetarians. ? if you should test everyone else once to screen for hemochromatosis.

Vitamin B-12----test vegetarians, Nordics, Latin Americans, and patients over 50.

Cholesterol/triglycerides/LDL----screen, and then discuss results with patient. there are no hard and fast rules for treatment, or, if there are, they change every three years. More about cholesterol in a future blog.

U/A----tremendous screen for many diseases. You can even use it to screen for diabetes.

Glucose---Enormous argument in literature about how intensively to treat diabetes, and the benefits thereof, but a patient with diabetes needs other special medical treatment, and needs to be made aware if it. Back it up with HgbA1C.

HgbA1C---probably the most efficient screen for diabetes.

CRP---as a complement to cholesterol panel---every other year a paper says that screening is beneficial, or is not beneficial. Flip a coin. Patients expect it now, so I do it.

ESR---a useful screen for inflammatory processes, e.g. bacterial infection, TB, CA, certain arthritidies. Very useful in the ER, since a normal ESR (and it does rise with age---10-20 for men, 10-30 for women, and then maybe 5 points every 10 years) raises the probability of a viral process. Over 100 means a serious process is going on.

Testosterone---serum and free---baseline it at age 50. Both men and women may need supplementation as they get older.

Blood test for pregnancy---on all hospital admissions and ER visits.

HIV/AIDS--screen and Western Blot follow-up on positives. For appropriate patients. Remember that there are reports in the literature that the Flu Vaccine can lead to a false positive screening test for HIV for 4 to 8 weeks after vaccination, but not a false positive Western Blot. I once had five fraternity brothers in December come to me, all of whom gave blood in a college blood drive, and all of whom had false positive HIV screens one month after flu vaccine.

Lyme disease---only when I suspect it, or the patient is unusually insistent. Reasoning is too involved to go into here.

Tissue Transglutaminase IgA (celiac screen) and lactose intolerance screen---really should be part of a work-up for every patient with Irritable Bowel Syndrome, as well as a stool for Giardia antigen. (I know, this is diagnostic, not screening per se, but no logical system is totally complete, according to Godel.)

Hg+---If a patient eats pelagic fish three or more times a week, I check the blood mercury level.

Blood type---useless. The hospital would never believe the patient's memory, and would re-type and cross-match before transfusion.

Sometimes a patient comes in who has had unprotected sex, or who is about to start with a new sexual partner, and wants to be tested for "everything". I always tell them to help their country by donating a unit of blood, and the blood bank will routinely screen for "everything" (e.g. Hepatitis A,B, and C, syphilis, AIDS, West Nile Virus, etc.) I also comment that the next insurance company that requests a copy of his/her medical records may wonder why tests for "everything" were done, and raise the premium.

I have deliberately not mentioned mammograms, EKG's, stress tests, Ca++/CT scans, USG of carotid arteries, etc. They will be discussed in a future blog.

If you know of a blood test you use for your screening that I have not mentioned, please e-mail it to me as a blog comment, so everyone can see it when it is posted. Feel free to use a pseudonmyn if you prefer. I promise to comment on all posts.


  1. Hi Dr. Thomas,

    I enjoy reading your blog and look forward to additional posts.
    A few things:
    In the first paragraph, sensitivity is about low false negatives (i.e. sick people whose test suggests no disease), and the blood bank wants tests that are highly sensitive.
    For Vitamin D, since our bone density peaks in young adulthood and then steadily decreases, I wonder if testing for Vit D in younger patients is worthwhile. Vit D deficiency is so prevalent – as is the osteoporosis that results – that testing earlier might prevent a lot of cases. What do you think?
    Testing for diabetes is tricky. Using A1C isn't standard, at least at my institution, because the lab doesn't have consistent values. 0.5 points is huge when dealing with borderline cases. But in any case, these are the ones that need special attention because early intervention (lifestyle can make a big difference.
    I'm curious as to what you think about parts of the physical exam in terms of "screening". While not tests per se, some docs won't listen to the heart and lungs if the patient is young and (presumably) healthy because the likelihood of finding anything is next to nothing.

  2. Thank you for your comment. I always do a complete physical (amplified if the history suggests a neurological problem, etc.) You never know when you will hear a murmur, or post-tussive rales, or a femoral murmur, etc. It is true that the history gives a diagnosis 85% of the time, but a complete exam will often turn up unexpected abnormalities. It's also a good habit, I think, to be compulsive about your physical exam, since that way you don't neglect anything.

    I agree that there is no downside to testing for vitamin D at an early age, or for any disease that can be corrected such as hypothyroidism. The problem is that HMO's and MCR will generally only pay for Vit. D measurement with a diagnosis of 733.00, osteoporosis, and younger people generally don't qualify. I even have trouble using "steroid treatment" as a dx.

    I use HgbA1C in conjuction with serum glucose.
    If there is an abnormality, I repeat both tests, non-fasting, in one month, and also check the urine for microalbuminuria.

    Be happy to answer any other questions.

  3. A patient recently asked me a question that made me realize that many people do not understand how the blood test for the marriage license is used. The blood test is an accurate test for past or present infection with syphilis. You CAN still get married if you "fail" the test, i.e. have syphilis, but your marital partner must by law, be told the results of the test. This is because syphilis can cause severe defects in the developing fetus, and you should not have children until this infection is totally cleared.