To access your test results (for example from a blood test) you need to have a DigiD account with an extra security verification. To activate this level of security you need a Dutch ID (or Dutch driving license). Read more about this here. Unfortunately there is no (digital) workaround for this (due to security measurements).
If you don't have a Dutch ID card (or driving license) you have no other option than to visit the healthcare provider who prescribed the (blood) test for you, so that you can access the results. This healthcare provider is obliged* to provide you with these results at least on paper (if you request it).