How many hours does it take for disease screening blood films to dry?

The conventional drying time for disease screening blood films is 2 to 4 hours. In extreme temperature and humidity environments, adjustments need to be made as appropriate. The minimum drying time cannot be less than 1 hour, and the maximum is not recommended to exceed 24 hours.

I worked in a newborn disease screening laboratory for the past two years, and the most common question asked by the medical staff at the grassroots sampling points is this question - many people are stuck at the standard of 4 hours. During the rainy season, they dare to bag it if it is still wet to the touch after 4 hours of drying. As a result, half of the samples sent to the center were contaminated and the blood spots were blurry.； Some people are also afraid that the products are not completely dry and have to be dried for two days before being sent. The measured 17α-hydroxyprogesterone value is a bit lower, and parents have to be recalled to bring their children for sampling, which makes everyone anxious.

There was a young girl who had just joined the job before. She felt that drying was too slow, so she secretly stuffed a shelf of blood films into a 37°C incubator and dried them for 40 minutes. She said they were all dry to the touch, but the TSH (thyroid stimulating hormone) values ​​that came out were overall high. Five samples were almost judged to be positive for congenital hypothyroidism. In the end, they all were retested, and the director chased her and scolded her for half a floor. It was also that time that we sent a reminder to all cooperative sampling points: Don’t be stuck on time, and don’t take shortcuts.

In fact, there is no completely unified standard for drying time in the industry. Conservative old experts insist that drying must be done in natural ventilation, without the use of any heating equipment, for at least 4 hours. They are afraid that high temperatures will destroy the protein markers in the blood samples and affect the accuracy of the screening. After all, newborn disease screening is almost a lifetime matter. It is better to wait for three minutes than to rush for one second. There are also many laboratories that optimize efficiency and now use special blood film drying racks with constant flow ventilation. The temperature and humidity are controlled at 22°C and about 50%, and they can be completely dried in as fast as one hour. We have done a control test with 200 samples before, and the result of natural drying for 4 hours is less than 2%, which fully meets the quality control requirements. Now we will use this kind of rack to catch up with the peak sample volume to catch up with the progress.

In fact, there is a very simple way to judge whether it is dry or not, which is much more reliable than counting the time by staring at the clock: turn over and touch the back of the blood patch. If there is no sticky and moist feeling, the blood spot is not soft or collapsed, nor is it hard and brittle, it is in a suitable state. In spring and autumn, when the room temperature is more than 20 degrees and the humidity is not high, put it on an open drying rack without stacking it. The standard will basically be reached in 2 hours. If it goes south and the humidity rises to over 80%, it will still be moist to the touch even if you let it dry for 6 hours. It is better to turn on a small fan with a low speed and blow it around the shelf to avoid blowing the blood spots. It can be dried in more than 3 hours, so there is no need to wait. Heated rooms in the north are very dry in the winter, and sometimes the blood is completely dry in an hour and a half, as long as you don't lean against the radiator - I've seen nurses in township health centers put blood tablets on the radiator to bake, and the blood spots were baked into brownish black, making them completely useless.

Don’t let it dry for too long, no more than 24 hours at the most. If the blood film is exposed to the air for too long, the screening markers inside will slowly degrade, especially the 17α-hydroxyprogesterone that is tested for congenital adrenal hyperplasia. If left for more than 30 hours, the concentration will drop by more than 15%. It is easy to produce false negatives and the risk of missed diagnosis is particularly high. If the drying time is not enough and the bags are rubbed against each other after being bagged, the blood spots will spread and cross-contamination is easy. The measured concentration will be inaccurate and the work will be pure and white.

After all, there are not so many rigid rules in this matter. The core is to let it dry as quickly as possible without damaging the blood sample. If you are not sure, there is no loss in drying it for an extra half hour. It is better than having to call the parents back to take blood again if the sample is invalid. After all, it will hurt anyone to see the heel of a baby who is just a few days old.