It's a review from Amazon:
Good pregnancy test, but users must be aware of the accuracy limits of the weeks estimator
By amo42 - December 9, 2014
There seems to be some confusion about what the results of this test mean. I'll try to clear things up. The 3 levels do in fact measure 3 different levels of HCG, but there's a huge range of sensitivity for each level. If your blood level is 250 and the test only shows 1-2 weeks, it doesn't mean the test is defective. What it does mean its that Clearblue's instructions are not really as clear as they should be.
The manufacturer has to file its test results with the FDA and that's public information. Check out the chart on page 6: http://www.accessdata.fda.gov/cdrh_docs/reviews/K112870.pdf
The Pregnant 3+ weeks threshold is 2600 mIU/ml. However if you look at their data, only 21 of 90 tests they did with exactly 2513 mIU/ml of HCG tested positive. The other 69 were false negatives. You aren't guaranteed to get a "3+ weeks" result until after your level is at least 4000. Clinical guidelines at that gestation are 18 - 7,340 mIU/ml, however, so basically this test can't tell you anything other than "you're pregnant" at 3 weeks past conception.
Please realize that this means that many many 4-week conceptuses (when your doctor would say you are 6 weeks along) are going to test as "2-3 weeks", and that is absolutely not a reason to panic. If you get one negative test, that does not mean a negative result. You have to do several tests, possibly even from different boxes, to be sure.
In summary:
If you get "Not pregnant", you may still have an HCG level below 17. Try another test today, and then again in 2 days.
If you get 1-2 weeks, that means that your HCG level is almost definitely between 10 and 500. It is most likely to be between 10 and 180.
1-2 weeks can turn positive anywhere above 10 ml/IU, but it doesn't reach 99% accuracy until 17. Clinical guidelines are 5 - 50 mIU/ml
If you get 2-3 weeks, that means that your HCG level is almost definitely between 120 and 4000. It is most likely to be between 180 and 2900.
2-3 weeks can turn positive anywhere above 120, but it doesn't reach 99% accuracy until 500. Clinical guidelines are 5 - 426 mIU/ml
If you get 3+ weeks, that means that your HCG level is almost definitely above 2100. It is most likely to be above 2900.
3+ weeks can turn positive anywhere above 2100, but it doesn't reach 99% accuracy until 4000. Clinical guidelines are 18 - 7,340 mIU/ml
The error bars on this test are wide enough that I would consider the weeks results to be entertainment only. You should absolutely not use this test to try to determine what your HCG level is. If your specific HCG level is important, go get a blood test done.
All that said, this is the only test on the market right now that can give you any clue at all about how much HCG is in your sample, and it can provide you some useful information. To know whether a particular level is accurate for a single sample, you'd need to take something like 10 tests at once. But if you just want reassurance that your pregnancy is progressing, you can use one test each time, and even with the large error bars on this test, it will eventually show progress, because your HCG level is steadily increasing. You may not hit "3+" until your 6th week however and that would still be within clinical guidelines. I wouldn't make any decisions based on this test unless the decision is to see a doctor and get a blood test done.