A six-year boy gets admitted to the hospital for fever. On routine investigations he is detected to have prolonged PT. Subsequent analysis revealed prolongation of PTT and Thrombin time as well. Normal plasma is added to patient's plasma in 1:1 ratio, but the clotting time does not get corrected. The child does not have any clinical bleeding.

The next investigation would be

1. Clotting factor assays
2. Euglobulin clot lysis time
3. Platelet function analyzer
4. Retiplase time
5. Lupus anticoagulant

Explanation:
If there is unexplained prolongation of PT, PTT, and Thrombin time, a mixing study is usually performed.
Normal plasma (which has abundant clotting factors) is added to the patient's plasma and PTT and PT is repeated.
Correction suggests clotting factor deficiency. Absence of correction suggests presence of an inhibitor (antibody). The most common cause of this finding in the inpatient setting is heparin contamination of sample.

The retiplase time uses a snake venom to clot fibrinogen. Unlike Thrombin time, it is not sensitive to heparin and is prolonged only by reduced or dysfunctional fibrinogen or fibrin split products and so helps distinguish the two.

Lupus anticoagulant produces similar picture but heparin contamination of sample is much more common.

Euglobulin clot lysis time tests the fibrinolytic system.

Platelet dysfunctions do not produce the said profile. They affect the BLEEDING TIME.