Most doctors will only tell you about abnormal lab test results and if nothing was what they consider abnormal they may only give you either a dumbed out summary or a verbal reassurance that "everything was fine."
Just how misleading this can be is shown by stories of people whose fasting blood sugar was 124 mg/dl who were told they were "fine", when one more mg/dl would have been enough to diagnose diabetes.
I experienced something similar. I was told my calcium values were fine when the normal range went up to 10.3 mg/dl and my reading was 10.3. Subsequent research turned up the fact that the high end of the "normal" range for blood calcium is associated with a significantly raised risk for heart disease.
As it was likely my raised blood calcium was due to overdoing my Vitamin D supplementation (which is a growing problem given the hype about this latest cure-all supplement) I stopped the supplementation since my blood Vitamin D level was far above the level defined anywhere as deficient and in another few months the blood calcium level had dropped to the middle of the range which is much less likely to cause serious health problems.
Keeping copies of truly normal lab results can also be important, because over time you may see a trend upward that points to a developing problem. If your "normal fasting blood sugar" each year was 75, 80, 85, 90, 95, 97, and 99 mg/dl these values would all be normal, but point to significant deterioration in your fasting blood sugar control. You'd end up a lot better off if you cut back on your carb intake while still normal than if you waited until it reached 101 mg/dl and you were officially told you were "pre-diabetic."
When you keep the actual copies of your lab tests you will also be able to tell if the doctor has actually done the right tests to determine what is wrong with you. For example, if you come from a family where many relatives have diabetes and have been relying on your doctor's assurance that you don't, it would be helpful to know whether he based this assurance on the fasting glucose test, the A1c, or a random glucose test.
If you are experiencing tingling in your extremities, your doctor should order a Vitamin B-12 test as Vitamin B-12 deficiency can look exactly like diabetic neuropathy and if not treated can cause permanent nerve damage. If you are taking metformin which can in some people cause problems with Vitamin B12 absorption and suddenly develop neuropathy while maintaining normal blood sugars, it is very important to ensure this test has been done.
If your doctor tells you that your cholesterol is too high, it is important to see whether this was based on a total cholesterol number alone, or took into consideration the readings for HDL and Trigycerides which are far more predictive of trouble.
Over the years, diagnostic standards change. This is another reason old lab tests may provide you with useful information years later. When I first ran into problems with my blood sugar, the definition of diabetes was a fasting blood sugar of 140 mg/dl. The cutoff subsequently dropped to 125 mg/dl.
A person who had been assured they weren't diabetic in 1996 based on the old standard might get more insight into why they already had diabetic complications like neuropathy or protein in urine the very day they were diagnosed with diabetes in 2000 if old copies of their fasting blood sugar tests showed they had been in the 130s consistently over the previous decade.
Another reason you want to hold on to your lab test sheets is that for many tests the range that defines normal varies from lab to lab and is not standardized. Labs also use different units for reporting test results. One of the big problems with Vitamin D recommendations is that labs use two different units for reporting blood levels of Vitamin D and people often assume they are deficient because they are looking at recommendations that are given using a unit that is not the one that their lab uses. Only your lab sheet will tell you what unit your lab used.
Insulin and C-peptide analysis are not standardized and and the lab reference ranges are different from lab to lab. Even the same hospital may send your blood draw out to different vendors for analysis so that the reference range will vary from year to year though you go to the same hospital lab. The reference range is essential for interpreting the result and when a test is not standardized, as is the case with insulin and C-peptide you can't compare your readings with those of others who use other labs.
There is another reason why you need to get copies of your test results right after you go to the lab. As I learned the hard way, after a few years it may be impossible to retrieve older lab results from the lab or your doctor.
Hospital labs do not keep results online for more than a few years. They are continually changing their computer systems and each time they make a change they get rid of old data. I found one set of labs impossible to retrieve from a local hospital only 3 years after they were done.
Doctors will have your old labs in their records, but you will lose access to those if you move to a new doctor. Even when you ask your old doctor to forward your medical records to the new doctor, what actually happens is that the doctor is sent a summary sheet not the actual records, which are put in storage somewhere and can be difficult or impossible to access if you need them.
If your doctor retires or moves to a new practice there may also be a brief period during which you can get your records, after which they may become impossible to retrieve.
When you get your lab tests you will often see values flagged as high that your doctor has not mentioned. The most common of these is the BUN which is often elevated in people who are eating a low carb diet because the low carb diet induces a degree of dehydration that affects the test result but does not cause any health problems. If you see a value on your lab sheet that concerns you, ask your doctor about it. There are also sites on the internet that explain the various test results and what they mean.
If you are paying for your lab tests as too many of us are, keeping copies of old tests can help you avoid unnecessary tests. If a doctor routinely schedules certain tests which you have long had normal values for, feel free to ask what the point of the testing might be.
All too often, the point of the test is to allow the doctor to qualify for some benefit to himself from imbecilic insurance-company sponsored programs that judge the quality of medical care by how many patients are given total cholesterol tests or quarterly A1c tests (irrespective of the value the tests show.)
If there is no health problem you need to diagnose or medication change you might make based on the result, there is no reason to pay for expensive testing.
When you go to a lab ask the person who does the test what the procedure is at that lab for getting your results. Some labs will give you the results as soon as they are ready if you show a driver's license. Others require you to fill out a form and will mail them to you.
It is helpful to get the results before your doctor's appointment so that you can discuss any issue you find in them. However, if you don't get a copy before the appointment, you can always ask the doctor to have a copy made at the front desk and can take that copy home with you.