Demystifying the Blood Drop A Hematopathology Primer on Peripheral Diagnostics

It appears your patient has a hematologic malignancy. The complete blood count (CBC) screams trouble — severe anemia, thrombocytopenia, and blasts. Immediate blood tests can start answering the urgent questions while the bone marrow biopsy is underway.

In previous Morning Reports, authors walked through bone marrow and blood smear diagnostics. Here, we continue with the blood draw, specifically stat flow cytometry and karyotyping/fluorescence in situ hybridization (FISH).¹ These tests provide provisional diagnoses, guide therapy, and let the hematopathologist triage test results efficiently.

The Basics

The laboratory performs four complementary analyses. Together they reveal morphology, immunophenotype, chromosomal architecture, and nucleotide-level lesions — potentially before the bone-marrow biopsy is processed.

Modality	What It Answers	Typical Turnaround*
Peripheral Smear	Are counts abnormal? Blasts or morphologic dysplasia?	Instrument: 30 minutes Manual smear: 2 hours
Flow Cytometry	What lineages are present? Are normal or aberrant antigens present? Is MRD detectable?	Same day: 4-8 hours
FISH/Karyotype	Are there hallmark translocations or aneuploidies?	FISH: 24-48 hours Karyotype: 5-7 days
PCR/NGS Panel	Which mutations or fusion transcripts are present?	RT-PCR: 4-6 hours NGS: 7-14 days

*Times vary by laboratory/blood draw timing but reflect common benchmarks.

Abbreviations: FISH, fluorescence in situ hybridization; MRD, minimal residual disease; NGS, next-generation sequencing; PCR, polymerase chain reaction; RT, reverse transcription.

Flow Cytometry

If the CBC and smear suggest blasts, flow cytometry (FC) provides rapid immunophenotyping and identifies cells that appear morphologically similar.² This process involves lysing red blood cells, staining leukocytes with eight to 12 fluorochrome-conjugated antibodies, and acquiring data for approximately 200,000 cells on a cytometer.³ Dot-plot or histogram analysis identifies cell lineage, maturation, and aberrant antigen patterns. Depending on the timing of the blood draw, turnaround is typically six to eight hours for STAT leukemia panels and 24 hours or less for comprehensive phenotyping. FC requires viable, unfixed cells in suspension and therefore provides no architectural context.

A) CD45 vs. side-scatter separating blasts (blue) and lymphocytes (red)
B) Myeloid panel showing CD34⁺/CD117⁺ blasts

FC rapidly answers critical questions including:

Lineage and maturation stage: distinguishes myeloid vs. lymphoid blasts, or clonally restricted B cells
Aberrant antigen expression: guide targeted therapy (e.g., CD33 for gemtuzumab)
Minimal residual disease: detect leukemic cells down to 0.01%

Quick tip: Low blood volume and prolonged transit are common causes of “insufficient event” reports and repeat phlebotomy.

FISH and Karyotype

Confirmation of abnormal blasts by FC leads to questions about chromosome appearance and underlying drivers of disease. Two assays provide answers at different magnifications:

Conventional karyotype involves culturing leukocytes for 24 to 48 hours, halting the cells in metaphase, G-banding the spreads, and assembling a 24-panel karyogram. This allows for viewing of balanced translocations, aneuploidy, and complex genomes (chronic myeloid leukemias [CMLs], many acute lymphoblastic leukemias [ALLs), constitutional studies).⁴
FISH skips culture entirely. The buffy coat is fixed to slides, fluorescent probes hybridize overnight, and dual-color signals light up target lesions (myeloid, chronic lymphocytic leukemia, myeloma)⁵

Figure 2. Cytogenetic and FISH detection of the BCR::ABL1 translocation

A) G-banded metaphase showing the Philadelphia chromosome t(9;22)
B) Dual-fusion BCR::ABL1 FISH in interphase nuclei

Image credit: Lina Shao, MD, PhD

Why order both?

FISH can quickly confirm a cryptic t(15;17) within hours, allowing immediate all-trans retinoic acid in acute promyelocytic leukemia, while karyotype reveals additional abnormalities that may upstage risk.
Karyotype may uncover unexpected anomalies that a limited FISH panel would miss (e.g., complex monosomal karyotype in AML).

Quick tip: Send an extra 3-5 mL of heparinized blood if CML/ALL is suspected to ensure enough metaphases are collected. Mitotic yield drops sharply after 24 hours.

Summary

A single blood draw can provide a detailed work-up for suspected hematologic malignancy. The peripheral blood smear offers an immediate morphologic snapshot; FC adds rapid immunophenotyping within hours; FISH and karyotype map chromosomal architecture in one to seven days; and PCR/NGS pinpoint driver mutations that influence risk stratification and targeted therapy. These modalities compensate for one another’s blind spots and can yield a provisional diagnosis.

Koutsi A, Vervesou E-C. Diagnostic molecular techniques in haematology: recent advances. Ann Transl Med. 2018;6(12):242.
Fang H, Wang SA, Hu S, et al. Acute promyelocytic leukemia: immunophenotype and differential diagnosis by flow cytometry. Cytometry B Clin Cytom. 2022;102(4):283-291.
Betters DM. Use of flow cytometry in clinical practice. J Adv Pract Oncol. 2015;6(5):435-440.
Ozkan E, Lacerda MP. Genetics, cytogenetic testing and conventional karyotype. In: StatPearls. StatPearls Publishing; 2025. Accessed June 23, 2025.
Gonzales PR, Mikhail FM. Diagnostic and prognostic utility of fluorescence in situ hybridization (FISH) analysis in acute myeloid leukemia. Curr Hematol Malig Rep. 2017;12(6):568-573.

Disclosure Statement: The authors indicated no relevant conflicts of interest.

Acknowledgment: This article was reviewed by Andrew Volk, PhD, and Henna Butt, MD.