White blood cells help your body fight infections. If you have HIV infection, your doctor may use a CD4:CD8 ratio test to track how well your immune system is responding to your medicine.

Some tests use other substances, such as blood or hair, to test for drugs. These may be more accurate in some cases, but the process is more complicated. Many administrators prefer to use urine ...

leading to inaccurate or invalid test results. See below to find out if your box of COVID-19 tests is still OK to use, as it's possible the expiration date has been extended. How do I know if the ...

To carry out an estradiol test, a medical professional will take a blood sample from a person’s arm or hand, clean the area, and look for a suitable vein. They will use a needle to draw blood ...

This test takes 15 to 30 minutes and is done while you’re reclining. The equipment used may vary. Some calorimetry tests use a mouthpiece you breathe into. Others require you to recline under a ...