Most laboratorians would agree that when it comes to hCG testing for pregnancy status, a blood sample is preferred over a urine sample. Why is that?
- Following implantation of a fertilized egg, hCG is first detected in the blood. hCG in the urine usually appears a day or two later.
- Blood-based hCG test results provide a concentration of the hormone whereas urine tests are qualitative (positive or negative)
- In early pregnancy, when hCG concentrations are still relatively low, a negative result can be obtained from a dilute urine sample (i.e. a false-negative result).
An earlier post presented the pros and cons of the different types of pregnancy tests. To summarize here: qualitative (yes/no) tests are typically easier to perform and, if urine is the sample being tested, then qualitative tests can truly be performed at the point-of-care. If a blood sample is used, it has to be centrifuged to separate the liquid part of the blood (serum) from the solid part (blood cells), a step that is nearly always performed in the laboratory. As such, qualitative hCG tests using a blood sample cannot be performed at the point-of-care.
Blood samples were collected from 632 women (aged 18-51 years) presenting to an emergency room. These blood sample were tested for hCG in two ways: 1) traditional, quantitative serum hCG testing in the laboratory and 2) qualitative, point-of-care hCG testing in the emergency room using the whole blood sample instead of serum. Note that the qualitative test used was not designed nor approved by the FDA for use with whole blood and that the test results were interpreted after 10 minutes whereas the test instructions required result interpretation after 5 minutes.
When compared to the results of the quantitative test performed in the lab, the qualitative test performed on whole blood (instead of serum) performed quite well. 100% of the laboratory-negative results were interpreted as negative using the qualitative test and 96% of the laboratory-positive results were interpreted as positive using the qualitative test. The hCG concentration of the 4% of "false-negative" results (9 out of 216) ranged from 5 to 50 IU/L. Notably, the hCG detection threshold of the qualitative test is identified as 25 IU/L.
So, what does all this mean? Well, the authors of the study suggest that it can be difficult to obtain urine samples from some patients which makes testing hCG in blood more attractive. But because blood-based hCG tests are normally always performed in the laboratory, waiting for the lab results can waste time. Hence, the appeal of testing whole blood for hCG at the point-of-care. The results of this limited study suggest that, perhaps, whole blood hCG testing using devices that were designed for urine or serum, has merit.
What the study did not address, however, are the (often) complicated state and federal rules that regulate lab testing. The study's use of a qualitative hCG test using whole blood instead of serum is problematic. Changing the type of sample recommended by the test manufacturer is a thorny issue for laboratorians who are ultimately responsible for all lab tests performed. It is typically the lab's responsibility to determine if an alternate sample type can be used for a given laboratory test and this type of substitution, however appealing, cannot be widely adopted without a thorough evaluation and validation by the laboratory.
Intriguing? Yes. Easily implemented? Not quite.