There are a lot of variables to consider here. The first is how much of the hCG that is injected ends up in the urine. Urine excretion of hCG is estimated at 10-12% of the injected dose over 24 hours. Keep in mind that this amount excreted will not be uniform and will vary based on the urine sample. So for a 150 IU dose of hCG, we could expect that about 10% of this, or 15 IU would get excreted over the duration of 24 hours. This is an absolute amount, vs. a concentration and so if a person has to urinate 5-6 times in a day it is impossible to know how much would be in any given void (this is separate from the issue of dilution). An average person urinates about 1-2 liters per day. So we have around 15 IU of hCG per 1-2 liters concentration, or 7-15 IU/L
An at home urine pregnancy test is sensitive to 25 IU/L for the most sensitive tests. For our example of a patient using 150 IU per day their urinary levels will be just below the sensitivity of the at home urine test. However, if they metabolize just a few percent more than average, or if their urine is slightly more concentrated, their urinary excretion levels would fall just within the sensitivity range for the test to go positive.
This surprisingly lines up very well with what is seen when people use the at home tests to test their hCG which is that the results vary greatly from the person and each time they would do it. Using this method is very prone to failing due to a huge margin of error based on your unique metabolism. The only way to be assured of the potency and purity of the product is to ask for a certificate of authenticity from the pharmacy which is a very rigorous standard (significantly higher level of testing than at home pregnancy tests) to ensure the product has what is listed on the label and nothing else, and also is as potent as advertised (these are two different standards which are not looked at with an at home pregnancy test).
Urine pregnancy tests are made to test hCG in a very different context and at very different urine concentrations which are also rapidly increasing day by day during a normal pregnancy. They have never been certified to be used for injected hCG at low doses and then metabolized to be measured in the urine.
Using the hCG solution from the vial directly onto a urine pregnancy test strip will have a concentration of 1000 IU/mL, or 1 IU/L Keeping in mind that the at home test is sensitive above 25 IU/L this will again only yield a positive result if enough of the test strip is saturated with a large volume of solution. The test strips are not designed to be used in this way, and errors from a phenomenon known as the ‘hook effect’ can yield false negatives from the antibodies on the test strip.
In summary, using the hCG hormone can sometimes cause a false positive on a urine pregnancy test, but generally the hCG dose levels will fall below that threshold. The results are unpredictable and will vary greatly. Dieters need to make sure there is no possibility of pregnancy during the diet because a false positive urine test has to be addressed as a true positive by the clinician until proven otherwise by a blood test and they diet has to be stopped for safety. The only way to establish the true potency, bioavailability and purity of the hCG is to request the certificate of authenticity from the pharmacy, or CoA.