Morning or evening blood test: does it matter?

Why the clock matters for some tests

Your body does not hold the same levels around the clock. Several hormones follow a daily rhythm, and other values move with meals, activity and body position. According to Vårdhandboken, the Swedish national handbook for healthcare, the time of day, medicines, diet and supplements can all affect test results.

For some values, the variation over the day is large enough to change how a result should be read. Laboratories handle this by recommending a set sampling time for exactly those analyses.

That is why the instructions for one test can say morning before ten, while another test can be taken whenever it suits you.

Values with clear diurnal variation

Cortisol is the clearest example. The value varies over the day, and according to 1177, Sweden's national health information service, the blood sample is usually taken in the morning because the value is at its highest then. When cortisol deficiency is investigated, care prefers a morning value taken around eight o'clock.

Testosterone also varies over the day. Karolinska University Laboratory recommends taking the sample between 7 and 10 in the morning because of the diurnal variation, and fasting, since the level can be lower for up to three hours after a meal. In investigations of testosterone deficiency, the sample is taken fasting in the morning after a normal night's sleep.

Iron moves a lot. Levels tend to fall over the day, and the variation within one person over a day is estimated at around 25 percent. Morning samples before 10 are therefore recommended, and iron tablets should not be taken the day before the test.

Values you can test at any time

HbA1c, which reflects your blood sugar over the last two to three months, can according to 1177 be taken at any time of day and needs no preparation. The value builds up over months, so a single morning or evening does not move it.

Hb, your haemoglobin, can also be taken at any time of day. There, other things than the clock matter, above all fluid balance and body position.

Whether a given test requires fasting, a morning sample or anything else is stated in the laboratory's sampling instructions. Fasting applies only to certain analyses and usually means ten hours without food or drink.

Why laboratories ask for morning samples

The reference interval, the span your value is compared with, is built on standardised conditions. According to Karolinska University Laboratory, most reference intervals are based on samples taken after 15 minutes of seated rest.

For values with diurnal variation, the morning sample serves the same purpose. When your sample is taken under the same conditions as the reference material, the comparison is fair, and when your own samples are taken at the same time of day, they can be compared with each other.

This is also why a value taken at the wrong time can look off without anything being wrong. An afternoon iron value often sits lower than a morning one, for purely physiological reasons.

Choosing a time in practice

If you are measuring hormones such as cortisol or testosterone, or iron, before 10 in the morning is the safe choice. Come rested, and fast if the instructions say so.

For values like HbA1c, the clock does not matter. Then it counts for more that the moment suits you.

If you are repeating a test to see change over time, take it at roughly the same time of day and under similar conditions as last time. Then you are measuring the change in you, and the day's normal swings stay out of the comparison.

This article is general health information, not medical advice and not an assessment of your values. If you have symptoms or concerns, contact your healthcare provider.