CAMLPR Hematology Practice Test

In ALL, especially the L1 subtype, the malignant cells are lymphoblasts and do not have the myeloid enzyme activity that MPO detects. Myeloperoxidase staining highlights myeloid lineage, so lymphoblasts in ALL typically show no MPO activity. That makes a negative MPO result the expected finding in ALL L1.

Sudan Black B is a myeloid stain for lipids in myeloid cells, so it’s usually negative in lymphoblasts as well, but the most consistently negative marker for ALL L1 among the options is MPO. Periodic acid–Schiff can be positive in lymphoblasts because of glycogen-rich cytoplasm, so a negative PAS is less typical. Platelet peroxidase relates to megakaryocytic/platelet lineage and is not a feature of lymphoblasts, so a positive result would not fit ALL L1.

Thus, the negative test result most characteristic of ALL L1 is the myeloperoxidase negative finding.