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

Step 1: Recall what coagulation needs.
Blood clots on an uncoated glass surface mainly through the intrinsic (contact activation) pathway of the coagulation cascade. Contact with a foreign, negatively charged surface like plain glass activates factor XII, which triggers a chain of clotting factor activations ending in fibrinogen being converted to fibrin, the mesh that forms a clot.
For this to happen, the sample needs an active supply of clotting factors, especially fibrinogen and prothrombin, still present and functional.

Step 2: Check fresh whole blood from an artery or a vein.
Fresh whole blood, whether drawn from an artery or a vein, still carries its full set of clotting factors and platelets. Placed on plain glass, contact activation proceeds normally, and the sample clots. Arterial vs venous origin does not change which clotting factors are present, only the oxygen content of the blood, so both (B) and (C) will coagulate.

Step 3: Check blood depleted of neutrophils alone.
Neutrophils are white blood cells involved in immune defence, not in the core clotting cascade. Removing only neutrophils leaves fibrinogen, prothrombin, platelets, and the rest of the clotting factors untouched, so this sample still clots normally on glass. So (D) will also coagulate.

Step 4: Check blood serum.
Serum is what remains after whole blood is allowed to clot and the clot, containing fibrin and cells, is removed. In other words, serum is blood plasma with the clotting factors, especially fibrinogen, already used up in forming the earlier clot.
Since serum no longer has usable fibrinogen or an active clotting cascade, placing it on glass again cannot trigger a fresh clot. There is nothing left to convert into fibrin.

Step 5: Compare all four options.
Options (B), (C), and (D) all still contain a working clotting cascade with fibrinogen intact, so all three coagulate on contact with plain glass. Only serum has already had its clotting factors consumed in an earlier clotting event, so it is the one sample that will not coagulate again.

Final Answer:
Blood serum will not coagulate on an uncoated glass slide, because its clotting factors are already spent.