Question

Which one of the following will not coagulate when placed on a glass slide that does not have any surface coatings?

• A. Blood serum
• B. Fresh blood obtained from an artery
• C. Fresh blood obtained from a vein
• D. Blood depleted of neutrophils alone

Hint:

Serum is what is left after blood has already clotted once, so ask what it no longer contains.

Answer 1

The Correct Option is A

This question checks whether you know the difference between blood plasma, blood serum, and whole blood, and how that difference affects clotting.

1. Blood serum: serum is produced by letting a blood sample clot completely and then removing the clot. Everything that clotting used up, most importantly fibrinogen, is gone from serum by definition. With no fibrinogen left to convert into fibrin, serum cannot form a new clot when it touches glass again.
2. Fresh blood obtained from an artery: arterial blood is oxygen-rich but still has its complete set of clotting factors and platelets untouched. Contact with bare glass starts the intrinsic pathway, through factor XII activation, exactly as it would in any fresh sample, so it clots.
3. Fresh blood obtained from a vein: venous blood differs from arterial blood mainly in oxygen and carbon dioxide content, not in clotting-factor content. It clots on glass for the same reason as fresh arterial blood.
4. Blood depleted of neutrophils alone: neutrophils belong to the immune system's white cell population and play no direct part in the fibrinogen-to-fibrin clotting cascade. Taking them out does not touch the clotting factors, so this sample clots normally too.

The only sample that has already burned through its clotting factors before it ever reaches the glass slide is serum, since by definition it is what is left after a clot has already formed once.

Let's summarize:

• Coagulation on bare glass needs fibrinogen and an intact intrinsic clotting pathway.
• Fresh arterial blood, fresh venous blood, and neutrophil-depleted blood all still have these intact.
• Serum is defined as blood with the clotting factors already consumed, so it cannot clot again.

So the sample that will not coagulate is blood serum.