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Important Time Intervals
The Timing of Common Medical Recommendations
Timing and Diagnosis
Time-Based Predictions
The Bottom Line

You see your doctor and he tells you:

See me again in 3 months.
Stop taking your anti-inflammatory medicine in 2 weeks.
Have a blood test in 6 weeks.

Did you ever wonder where these time intervals came from? Are they really essential?

Timing is important in medicine. For example, you may have heard of the "golden hour." For certain types of major trauma, it's essential to get medical attention within an hour to improve the chances of survival.

There are many other common situations in medicine when time is important. But then there are occasions when timing is much less critical and the recommendations tend to be much more arbitrary.

If you have the time, we'll review some of these situations now.

Important Time Intervals

In addition to "the golden hour," there are other time intervals that may be important to keep in mind. For example:

4 minutes – When the heart stops or someone stops breathing, it's important that CPR start within a few minutes. If more than 4 minutes go by without supporting or restoring circulation or breathing, brain damage commonly occurs and survival rates drop dramatically.

2 hours – Getting medical treatment (including powerful blood thinners) for heart attack within 2 hours is associated with better survival rates than later medical care.

3 hours – People with stroke symptoms (such as sudden weakness on one side of the body or trouble speaking) have better outcomes if they get treatment (again, including blood thinners), within 3 hours of their symptoms.

4 days, 40 days – The general guideline for disasters is that a person can survive about 4 days without water, and about 40 days without food. There are many exceptions because other factors also affect survival, such as a person's overall health before the disaster.

The Timing of Common Medical Recommendations

Some medical recommendations have time intervals that are based primarily on what is easy to remember. Some common ones are:

Yearly physical examinations – There is no biological function or disease process that is correlated with the time it takes the earth to travel around the sun. That includes the routine physical examination. In fact, a number of studies have called into question the value of yearly examinations.

Antibiotic use – You may have noticed that when you take antibiotics, it tends to be for 5 days, 7 days or 10 days. These intervals are based on research studies showing that antibiotics are highly effective for certain infections when taken for these time periods. But few studies compare taking the same antibiotic for the same condition for 4 days, 6 days or other lengths of time. It's quite possible that a shorter course of treatment (or even a longer one) would be just as effective or even better than the standard interval. However, until additional studies prove it (and recommendations change), it's best to take your prescribed antibiotics according to directions and finish them.

Bone mineral density testing – This standard test for osteoporosis is repeated every year or two for many people. But there's little evidence that such an approach is helpful. Bone density tends to change slowly, even with treatment. The ideal interval for repeating this test is uncertain. One recent study suggested that much longer intervals — up to 15 years — would be adequate for people at low risk of osteoporosis.

Mammograms – Recommendations about when to start and how often to repeat mammograms have been controversial for years. But, once screening begins, the usual recommendation is to have the test every year or every other year, not every 18 months or 32 months or some other interval. Researchers typically choose 1- or 2-year intervals to study because they are convenient and easy to remember, rather than anything specific about the growth or spread of breast cancer. Similar principles apply to colonoscopy, pap smears and other common screening tests.

Timing and Diagnosis

Knowing how symptoms vary over time and when they started can help a doctor figure out what you have. For example, abdominal pain that's been coming and going for years is likely to have a different cause and treatment than new abdominal pain that has just suddenly developed. It helps a doctor to know whether your pain started after an injury, exercise or surgery. A rash that developed after starting a new medication could be an allergic reaction; but if you had the rash before you started the medicine, the new medicine is probably not to blame.

In addition, time can be used as a diagnostic test. For example, joint pain often disappears just as mysteriously as it appears. Rather than undergoing extensive testing, it's often better to give it some time (and take some acetaminophen or ibuprofen).

Time-Based Predictions

Patients often assume doctors can make accurate predictions about prognosis or survival. But, "giving a patient 6 months to live" is something you hear in movies or television much more often than in real life; doctors can rarely make such predictions with confidence. (Perhaps that's why you sometimes hear about people getting a dire prognosis and then outliving their doctors.)

Prognosis can be estimated based on studies and doctors' experience. So a doctor might predict a short survival based on what's reported in the medical literature or other patients he or she has treated. But survival tends to vary greatly. For example, a person with pancreatic cancer may be told he has 6 months to live based on average life expectancy of all people with this type of tumor; but they should also be told that it varies from a few weeks (for advanced cases) to several years or more (with tumors that are small, haven't spread or respond well to treatment).

Yogi Berra once said, "It's tough to make predictions, especially about the future." When it comes to making long-term medical predictions, I think doctors (and patients) should take this to heart.

The Bottom Line

Clearly, time matters in medicine. Significant trauma, heart attack and stroke should be treated as quickly as possible. And, the timing of symptoms can help sort out the diagnosis.

Perhaps your doctor wants to recheck your blood pressure in 3 months to give your weight loss program and low salt diet a chance to work.

Maybe the blood test is in 2 weeks because it takes that long to see the effects of your new thyroid medicine. The anti-inflammatory medicine can damage the lining of the stomach, so stopping it in a month might limit its impact. There can be good reasons for time-based recommendations. But often, there aren't.

When there is no single best amount of time for a particular recommendation, your input matters. For many of my patients who need a follow-up appointment, I've started asking, When would you like to come back?" Sometimes they suggest coming back sooner than I would have recommended, sometimes later. Regardless, time intervals in medical care can often be negotiated rather than chosen only by the doctor.

Sometimes, timing is not everything. If you have a question about the timing of your symptoms or your medical care, be sure to ask your doctor.

Robert H. Shmerling, M.D. is associate physician at Beth Israel Deaconess Medical Center and associate professor at Harvard Medical School. He has been a practicing rheumatologist for over 20 years at Beth Israel Deaconess Medical Center. He is an active teacher in the Internal Medicine Residency Program, serving as the Robinson Firm Chief. He is also a teacher in the Rheumatology Fellowship Program.

Why Timing Is Important in Medicine