What causes blood to be unusable for a blood-glucose meter within mere seconds?

If I draw a pinprick of blood and promptly load it onto a test strip in a blood-glucose meter, it gives a valid reading.

But a mere 10 seconds later, that same drop of blood on my finger would give an error message when put on a test strip in a meter.
Is it that the blood is clotting rapidly, and something about the clotting makes the machine unable to read blood sugar? Does the blood sugar itself change in that droplet of blood as it clots?

Don’t have a complete answer but some clues from wifey who has worked in healthcare. She is trained in blood draws, and has cared for a diabetic cat for years.

She says blood starts “agglutinating” (coagulating) very quickly after it leaves the body. Meanwhile, she’s not sure how the blood sugar is measured, but these devices might be measuring something different, i.e., the blood sugar measurement is indirect. So the rapid coagulation is possibly affecting the blood sugar measurement somehow.

Modern at home blood glucose meters operate by reducing the sample with an electrode made of dehydrogenase, which is reoxidized by an intermediate mediator, which itself is reoxidized by a charged electrode. The total charge required is measured against a calibrated value to give an estimate of blood glucose level. If the water in the serum is removed through dehydration (which occurs during coagulation after exposure to free air) then the impedance changes, screwing up the measurement. Lab-based testing actually separates whole blood into components and just measures the glucose levels in serum without platelets and blood cells, which is far more accurate but takes time and special equipment.