•Ultradeep sequencing of immunoglobulin genes can identify residual disease.•Low-level detection of clonally rearranged immunoglobulins is a stochastic process.•A sufficient amount of DNA must be analyzed to detect one cell in a million.•Standardization of measurable disease detection by sequencing is found imperative. Malignant lymphoproliferative disorders collectively constitute a large fraction of the hematological cancers, ranging from indolent to highly aggressive neoplasms. Being a diagnostically important hallmark, clonal gene rearrangements of the immunoglobulins enable the detection of residual disease in the clinical course of patients down to a minute fraction of malignant cells. The introduction of next-generation sequencing (NGS) has provided unprecedented assay specificity, with a sensitivity matching that of polymerase chain reaction-based measurable residual disease (MRD) detection down to the 10–6 level. Although reaching 10–6 to 10–7 is theoretically feasible, employing a sufficient amount of DNA and sequencing coverage is placed in the perspective of the practical challenges when relying on clinical samples in contrast to controlled serial dilutions. As we discuss, the randomness of subsampling must be taken into account to accommodate the sensitivity threshold—in terms of both the required number of cells and sequencing coverage. As a substantial part of the reviewed studies do not state the depth of coverage or even amount of DNA in some cases, we call for increased transparency to enable critical assessment of the MRD assays for clinical implementation and feasibility.