Preparing for laboratory examinations

Many factors—such as eating, physical exertion, stress, coffee, tobacco, alcohol, and medications (including herbal products and vitamins)—can affect laboratory examinations results. Avoid smoking, alcohol, and strenuous physical activity during the 24 hours prior to sample collection.

Immediately before sample collection, it is recommended to sit calmly for about 15 minutes to allow blood circulation to stabilize.

Some laboratory examinations require fasting before sample collection. Check your patient instructions to see how to prepare for the examination. For a fasting sample, you must refrain from eating for 10–12 hours. During fasting, you may drink a glass of water.

The letter f at the beginning of a test name means that fasting is required before the test (for example, fP-Glucose). The specimen types used in blood sample testing may vary by region. If plasma is used as the specimen, the test prefix is fP. If serum is used, the prefix is fS.

Certain laboratory examinations require sample collection at a specific time of day. In these cases, your physician or nurse will instruct you to come to the laboratory at the designated time. 

Medications can usually be taken as normal before laboratory examinations unless otherwise instructed by your physician. If a blood sample is taken to assess the concentration of a specific medication, take the medication only after the blood sample has been collected, unless your treating physician provides different instructions. The sample collector may ask about your medication use before sample collection. 

For the examination of cells or bacteria in urine, the urine should remain in the bladder for at least 4 hours before sample collection. This allows the substances being tested to concentrate in the urine, resulting in more reliable test results. 

If you have received supplies and instructions from the laboratory for collecting a urine sample at home, be prepared to report the time of sample collection and the duration (in hours) that the urine remained in the bladder before collection when you bring the sample to the laboratory. 

Some urine examinatios require timed collection, where all urine is collected over a specific period. You will always receive separate instructions and supplies for these examinations.