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Content of Prenatal Tests

From ob-gyn-l:

I am joining a small HMO with large local market share, and have to make many changes about which I am not entirely comfortable. Two regard prenatal lab evaluation:

1. AFP is a double-screen, not triple, excluding estriol;
2. No "routine" urine culture is done, just a "routine" urinalysis, with culture ordered for "dirty" U/A's only if the patient is symptomatic, when called by the nurse.

I still use triple screen..

Asymptomatic bacteriuria is a condition for which treatment is required. I like to get a urine culture on everyone. How sensitive is a "dirty urine" for detecting asymptomatic bacteriuria?

What about PPDs and tetanus toxoid? Are you doing them? Which patients?

For me, my patient population is mostly indigent and high risk. I screen with PPD all foreign born (from areas of high incidence of TB), HIV +, health care workers, and close contacts of a case of TB.

Anyone bothering with 2 step testing?

Tetanus toxoid for those without immunization for 10 years..

According to recent CAP surveys, >75% of all labs in the US do triple tests, and >90% of patients who undergo multiple mark screening (some just have MSAFP - e.g., those who had CVS) have triple testing done.

It is likely that your HMO has a capitated contract with a lab which just offers a double test (e.g., MSAFP + MShCG or MS "free-beta" hCG). For all labs, the added cost of estriol testing increases costs. Most labs (the ones which do triple testing) absorb the added costs of analyzing estriol, but the others can't seem to justify the additional expense. IMHO (and those of most other centers), the cost is justified by the preponderance of data in the scientific literature - particularly if one is concerned about detection rates for trisomy 18.

A case can be made for the double screen with AFP and HCG. You will just have a slightly lower sensitivity, while having a slightly higher specificity. i.e. you will do a lot less amnios while missing only a few more trisomys.

Actually, we do less amnios with the triple test than with the double. I realize that this may not be the case for all laboratories where it is their best financial and/or medicolegal interest to have more positive screens. We anticipate that both the specificity and sensitivity will increase further once we move to a quad test.

My understanding is that a double screen is "almost" as good as a triple. I wouldn't be too worried about that. We don't routinely do urine cultures in our practice, only if the UA shows bacteriuria.

2 step testing is described in the CDC's MMWR Recommendations and Reports Vol 44 Number RR-11 Titled: Essential Components of a Tuberculosis Prevention and Control Program, Screening for Tuberculosis and Tuberculosis Infection in High-Risk Populations.

See: http:

2 step testing is used mainly by institutions as the first test in a series for those who will get frequent screening as a result of their ongoing exposure to TB (health care workers or institutionalized persons). It identifies the booster phenomenon. This phenomenon occurs in people with a prior history of BCG or infection a long time ago where the delayed-type hypersensitivity has waned. The PPD can initially be negative, but up to 1 year later it can stimulate a positive response to a subsequent skin test. Those people would falsely be labeled as converters.

Two step testing (2 PPD's 1 to 3 weeks apart if the first is negative):

                     first test        second test
positive          pos                  N/A
booster           neg                   pos
negative          neg                   neg