Maybe baby, but no wee? Use whole blood on urine β-HCG panels

In a recent post on an approach to "The First Five minutes of Resus", I saw mention of using a urine pregnancy test with whole blood to quickly screen for pregnancy.

There are many situations where pregnancy-related problems, or even just knowing the patient is pregnant, would quickly alter our management. In many resus cases, however, getting a urine sample is not immediately possible, and getting a blood result back from the laboratory could easily take an hour. We already routinely take off blood for full testing, including point of care tests such as BSL, Hb, lactate and venous gas analysis.  Can we also reliably use the urine pregnancy panel as another rapid bedside test by using whole blood instead of urine?

The answer, it seems, is likely to be yes. Some urinary panels are already approved for use with serum - it would be interesting to see if those in use in your department are too. It is likely, however, even if not approved for blood use, that the panels will be accurate at detecting positive results, with the caveat that a negative result does not entirely exclude pregnancy (then again, neither does a negative urine test).

There is a good summary at ALiEM, based on a study published in the Journal of Emergency Medicine. They reported results (after a 10-minute wait) similar to those using urine samples:

  • Sensitivity 95.8%
  • Specificity 100%
  • Negative predictive value 97.9%
  • Positive predictive value 100%

An earlier case report of the use of whole blood drew a response from laboratory-based pathologists, stating that acting on a false positive could do harm and that:

Although it can be tempting to adapt the intended use of diagnostic tests to fit specific clinical needs, to do so without input and guidance from clinical laboratory professionals is risky and unlawful.

Apparently to use a diagnostic test, such as the urine pregnancy test, in a manner not approved by the FDA without validation, is illegal. The authors of the case report respond:

We have always found it wise to be measured in our allegations of illegality, particularly when referring to a physician's scope of practice rather than regulations that apply to an accredited laboratory.

A new bedside test for resus?

So should we be using this as a quick bedside test in our resus rooms with a few drops of our routine blood samples, when pregnancy may alter our immediate management?

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