ABSTRACT Two main techniques are being used for the detection of minimal residual disease (MRD) in acute leukemia (AL): immunophenotypic analysis and polymerase chain reaction (PCR). In this paper, we analyze the results of assessing MRD by means of flow cytometry (FC) in a group of 93 patients with AL who were prospectively studied and treated in a single institution over a 9-year period. The presence or absence of MRD was established at a cut-off level of 2%, as judged by FC; a single result above this level was considered to define the positivity. The patients were grouped in 4 subsets: (1) acute lymphoblastic leukemia (ALL) patients with MRD (n = 36); (2) acute myeloblastic leukemia (AML) patients with MRD (n = 13); (3) ALL patients without MRD (n = 31); and (4) AML patients without MRD (n = 13). The relapse rates in these groups were 17%, 8%, 0%, and 0%, respectively, whereas the overall 7-year survival was 65%, 69%, 83%, and 98%, respectively. Our results support the usefulness of serially assessing MRD in patients with AL by means of FC; because this method is applicable to all cases of AL, despite being less sensitive than a molecular biology study, it is a good option to follow-up patients and to decide therapeu- tic and timely interventions. Lab Hematol. 2007;13:xx-xx. KEY WORDS: Minimal residual disease • Acute leukemia • Follow-up • Flow cytometry INTRODUCTION Morphologic detection of low-level residual acute leukemia (AL) is insensitive to changes in disease burden and therefore not a suitable method for monitoring continued disease response or early relapse below the standard threshold of 5% blasts as judged by conventional morphology. Patients in morphological “remission” may still have up to 10 10 resid- ual leukemic cells [1,2]. Furthermore, there is an established clinical importance for detection of low-level residual disease, with a number of studies showing prognostic significance when the disease burden is as low as 0.01% and higher for AL [2]. Diagnostic morphological techniques, routinely used in clinical practice for monitoring AL patients, are able to detect only 1% to 5% of malignant cells. At present, 2 main techniques are being used for the detection of minimal resid- ual disease (MRD) in AL, namely immunophenotypic analy- sis and the polymerase chain reaction (PCR) technique with a general sensitivity of 10 –4 to 10 –5 . Immunological marker analysis allows detection of unusual and aberrant immunophenotypes and is usually performed by flow cytom- etry (FC) [1,2]. PCR analysis allows the detection of leukemia-specific DNA sequences, such as fusion regions of chromosome aberrations and junctional regions of rearranged immunoglobulin (Ig) and T-cell receptor (TCR) genes [1,2]. The applicability of the immunophenotyping and PCR-mediated MRD techniques is dependent on the Minimal Residual Disease Testing in Acute Leukemia by Flow Cytometry Immunophenotyping: Prognostic Significance GUILLERMO J. RUIZ-ARGÜELLES, 1,2 DANITZA FERNÁNDEZ-LARA, 1,3 ROBERTO ESTRADA-GOMEZ, 2 CARLOS MANZANO, 1 GUILLERMO J. RUIZ-DELGADO, 4 BEATRIZ PÉREZ-ROMANO, 2 ALEJANDRO RUIZ-ARGÜELLES 2 1 Centro de Hematología y Medicina Interna de Puebla, Puebla, Mexico; 2 Laboratorios Clínicos de Puebla, Puebla, Mexico; 3 Hospital Angeles de Interlomas, México City, Mexico; 4 Hospital Universitario, Universidad Autónoma de Nuevo León, Monterrey, Mexico Received November 27, 2006; received in revised form January 30, 2007; accepted February 8, 2007 Laboratory Hematology 13:xx-xx © 2007 Carden Jennings Publishing Co., Ltd. doi: 10.1532/LH96.06045 1 Correspondence: Guillermo J. Ruiz-Argüelles, MD, FACP, FRCP (Glasg), Centro de Hematología y Medicina Interna de Puebla, 8B Sur 3710, 72530 Puebla, Mexico; 52-222-243-8100; fax: 52-222-243- 8428 (e-mail: gruiz1@clinicaruiz.com).