Diagnostic Tests: Auer Rods


Overview — What This Is


The structure shown is an Auer rod, which is an eosinophilic, needle-shaped cytoplasmic inclusion found within myeloblasts or promyelocytes. These inclusions are composed of abnormal azurophilic granules and represent crystalline aggregates of myeloperoxidase. Their presence indicates a malignant proliferation of myeloid precursors.


Disease Associated With Auer Rods


Auer rods are pathognomonic for acute myelogenous leukemia (AML).
They may appear in several AML subtypes, but are especially prominent in:
• Acute Promyelocytic Leukemia (APL, M3) — often showing bundles of Auer rods (“faggot cells”), which is a high-yield test clue.


Their presence confirms the myeloid origin of the malignant blasts


When This Is the Correct Answer


You should answer Auer rods / AML when the question presents a patient with:
• Pancytopenia (fatigue, infections, bleeding)
• Peripheral smear showing blasts
• >20% blasts on bone marrow biopsy, meeting diagnostic criteria for acute leukemia
• Systemic symptoms such as fatigue, fever, infections, bruising, or bleeding


Additional diagnostic support includes:
• Myeloperoxidase (MPO) positivity on histochemical staining
• Expression of myeloid markers: CD13, CD33, CD117
• Presence of Auer rods confirms AML over ALL or other hematologic malignancies


In APL (t(15;17)), Auer rods are particularly dense and clinically important because the disease is associated with DIC, and treatment requires all-trans retinoic acid (ATRA).

Diagnostic Tests: Fragmented Red Blood Cells (Schistocytes)​

Overview — What This Is

The smear shows fragmented red blood cells, also referred to as schistocytes or helmet cells. These irregularly shaped RBC fragments form when erythrocytes are mechanically sheared within the circulation. Their presence is a hallmark of intravascular hemolysis and defines microangiopathic hemolytic anemia (MAHA).


Diseases Associated With Schistocytes

Fragmented RBCs occur in conditions where red cells are destroyed while passing through areas of fibrin deposition, turbulence, or mechanical injury. Major associated disorders include:

• Thrombotic Thrombocytopenic Purpura (TTP)
• Hemolytic Uremic Syndrome (HUS)
• Disseminated Intravascular Coagulation (DIC)
• Major blood group incompatibility (e.g., ABO mismatch)
• Paroxysmal Nocturnal Hemoglobinuria (PNH)
• Mechanical destruction from:
  • Artificial heart valves
  • Mechanical circulatory support devices (e.g., LVADs)


• Snake bites (due to hemotoxic venom effects)

These conditions share a common mechanism—red cell fragmentation within the vascular system.


Associated Laboratory Abnormalities

Because schistocytes signify intravascular hemolysis, several classic lab abnormalities accompany their presence:

• Elevated LDH — released from destroyed RBCs
• Elevated indirect (unconjugated) bilirubin — from heme breakdown
• Elevated reticulocyte count — bone marrow compensation
• Decreased haptoglobin — binds free hemoglobin, becomes depleted in intravascular hemolysis

These findings support the diagnosis of hemolysis and help distinguish MAHA from other causes of anemia.

Diagnostic Tests: Spherocytes on Peripheral Smear

Overview — What These Are

The peripheral smear shows spherocytes, which are small, dense red blood cells lacking the normal central pallor. Their spherical shape results from loss of membrane surface area, making them less deformable and more prone to destruction, especially in the spleen. Spherocytes are a hallmark of hereditary spherocytosis, but can also appear in immune hemolysis.


Most Accurate Diagnostic Test

The most accurate diagnostic test for hereditary spherocytosis is the osmotic fragility test.
This test demonstrates that spherocytes lyse more easily than normal RBCs when placed in hypotonic solutions due to their decreased membrane surface area.


(Note: An elevated MCHC is supportive but not the most accurate test.)


Genetic Defect

Hereditary spherocytosis is caused by defects in red blood cell membrane proteins, most commonly involving:

• Ankyrin gene mutation → leading to
• Spectrin deficiency → which
• Produces membrane instability and premature RBC destruction (extravascular hemolysis)

Other possible but less common defects include band 3 and protein 4.2, but ankyrin/spectrin is the classic board answer.


When to Choose This Diagnosis/Test

You should think of hereditary spherocytosis—and choose osmotic fragility as the test—when the case involves:

• Recurrent hemolysis
• Splenomegaly (often marked)
• Family history of anemia (autosomal dominant pattern in most cases)
• Increased MCHC on CBC (a high-yield clue)
• Spherocytes on peripheral smear
• Signs of hemolysis: jaundice, increased reticulocytes, gallstones (bilirubin stones)

These findings together strongly support hereditary spherocytosis as the diagnosis.

Diagnostic Tests: Target Cells

Overview — What These Are?

The cells shown are target cells (codocytes). Target cells have a characteristic “bull’s-eye” appearance caused by an excess of cell membrane relative to cell volume. This altered surface-area–to–volume ratio creates a central area of hemoglobin surrounded by a pale ring and then an outer ring of hemoglobin.

Most Accurate Diagnostic Test

The most accurate diagnostic test when evaluating conditions associated with target cells is:

Hemoglobin Electrophoresis

This test helps identify hemoglobinopathies such as:

• Sickle cell disease
• Hemoglobin C disease
• Thalassemia

Electrophoresis will distinguish between different abnormal hemoglobins and determine specific genetic variants.

Associated Diseases

Target cells may appear in several hematologic and systemic conditions, including:

• Hemoglobinopathies
  • Hemoglobin C disease (most strongly associated)
  • Sickle cell disease (HbSS)
  • Thalassemias (α and β)


• Liver disease
  Due to increased membrane lipids, resulting in excess surface area.
• Iron deficiency anemia
  Though less classic, target cells may be seen in severe cases.

Their presence should prompt consideration of hemoglobin abnormalities or liver dysfunction.


Do Target Cells Have a Shorter Survival?

Despite their abnormal morphology, target cells do NOT have a shorter survival than normal red blood cells.
They are structurally abnormal but not inherently more fragile, unlike spherocytes or sickled cells.

Diagnostic Tests: Sickled Cells

Overview — What This Is


The image shows sickled red blood cells, which are elongated, crescent-shaped RBCs formed due to polymerization of abnormal hemoglobin (HbS). Sickled cells are rigid, fragile, and prone to causing vaso-occlusion and hemolysis.

Diseases Associated With Sickled Cells

Sickled cells are seen only in:

Sickle Cell Disease (HbSS — Homozygous)

• Patients with homozygous hemoglobin S (SS) produce sickled cells, especially during episodes of dehydration, acidosis, hypoxia, or infection.
• These patients exhibit hemolytic anemia, vaso-occlusive crises, acute chest syndrome, splenic infarction, priapism, and other complications.

Important distinction:

• Sickle Cell Trait (AS — Heterozygous) does not produce sickled cells on a routine peripheral smear.
  Individuals are generally hematologically normal, though they may sickle under extreme conditions (e.g., severe hypoxia).

​

Most Accurate Diagnostic Test


The most accurate test for diagnosing sickle cell disease is:


Hemoglobin Electrophoresis


This test identifies and quantifies hemoglobin variants and will show:

• HbS predominates in sickle cell disease (SS)
• A mixture of HbA and HbS in sickle cell trait (AS)

Electrophoresis provides definitive confirmation and distinguishes SS from AS and other hemoglobinopathies.

Diagnostic Tests: Rouleaux Formations

Overview — What These Are

The structures shown are rouleaux formations, in which red blood cells stack together like coins. This stacking occurs when plasma proteins reduce the normal negative charge (zeta potential) between RBCs, allowing them to adhere to one another. Rouleaux formation is a nonspecific finding but often reflects elevated levels of certain plasma proteins.


Diseases Associated With Rouleaux Formation

Rouleaux commonly develops in conditions with increased abnormal globulins or elevated fibrinogen, including:


1. Multiple Myeloma

• Overproduction of monoclonal immunoglobulins (especially IgG) promotes RBC stacking.
• Supporting clues include:
  • Anemia
  • Hypercalcemia
  • Renal failure
  • Lytic bone lesions
  • Abnormal SPEP (serum protein electrophoresis) or UPEP (urine protein electrophoresis) showing M-spik
    
    
  
  

2. Waldenström Macroglobulinemia

• Characterized by high levels of IgM macroglobulin, which greatly increases blood viscosity.
• Also causes rouleaux due to elevated globulin levels.

Other possible but less specific associations

• Chronic inflammation
• Connective tissue disorders
• Certain infections
• Conditions with elevated fibrinogen (e.g., acute phase reactions)

But on board-style exams, multiple myeloma and macroglobulinemia are the classic associations.
​

Diagnostic Tests: Ringed Sideroblasts



Overview — What This Is

The cell shown is a ringed sideroblast, a pathological erythroblast with iron-loaded mitochondria encircling the nucleus. These iron deposits can only be seen clearly using a Prussian blue stain, which highlights the mitochondrial iron in a ring-like pattern. Ringed sideroblasts reflect a failure of the developing red blood cell to properly incorporate iron into heme during erythropoiesis.

What Causes This?


Ringed sideroblasts form when iron accumulates in mitochondria because of defective heme synthesis within erythroid precursors. This impairment leads to:

• Microcytic anemia
• Elevated serum iron level
• Ineffective erythropoiesis

Common causes of acquired sideroblastic anemia include:

• Alcohol use (most common)
• Drugs, such as:
  • Isoniazid (INH)
  • Chloramphenicol


• Toxins, including:
  • Lead
  • Zinc

These agents disrupt enzymes required for heme synthesis, especially ALA synthase, resulting in mitochondrial iron trapping.

Associated Hematologic Disorders

Ringed sideroblasts are the defining feature of sideroblastic anemia, but they are also associated with other disorders, including:

• Myelodysplastic Syndrome (MDS) — particularly the subtype known as refractory anemia with ring sideroblasts (RARS)
• Congenital sideroblastic anemias (less common, involve mutations in heme synthesis enzymes)

Thus, their presence should prompt evaluation for toxin exposure, alcohol use, medications, and underlying marrow disorders such as MDS.


If you’d like, I can create a comparison of sideroblastic anemia vs. iron deficiency anemia or provide a quick diagnostic algorithm for microcytic anemias.

Diagnostic Tests – Papanicolaou (Pap) Smear

1. What Is a Pap Smear?

The Papanicolaou (Pap) smear is a cytologic screening test used to evaluate cells taken from the cervix in order to identify early precancerous changes or established cervical cancer. By examining exfoliated cervical cells under a microscope, clinicians can detect abnormalities caused primarily by high-risk HPV infections long before they progress to invasive disease. The Pap smear is one of the most effective screening tools in medicine and has significantly reduced cervical cancer mortality in populations where it is routinely used.


2. When Is Pap Smear the Correct Answer?

A Pap smear is indicated as a routine screening test for cervical cancer. Screening begins at age 21, regardless of sexual history, or earlier if guidelines specify onset within three years of first sexual activity; however, age 21 is the standard U.S. board-style answer. The test should be repeated every 2–3 years, depending on the screening strategy and incorporation of HPV co-testing. Screening may be discontinued at age 65 if the patient has had a history of adequate, consistently negative prior screenings and no recent abnormalities. Pap testing is used specifically for asymptomatic women as part of preventive care.


3. What Is the Next Best Test If Results Are Positive?

When a Pap smear shows abnormal cytology—whether atypical cells, low-grade changes, or high-grade lesions—the next step is to perform a colposcopy, during which the cervix is carefully examined under magnification. Suspicious areas are then biopsied to obtain tissue for definitive diagnosis. Colposcopy provides direct visualization of the transformation zone, allowing clinicians to identify the exact location and severity of cervical lesions and to determine appropriate management based on biopsy results.