Diagnostic Tests: Cold Agglutinins

Overview


Cold agglutinins are IgM autoantibodies directed against antigens on red blood cells. These antibodies become active at lower temperatures, typically between 0–4°C, causing RBCs to clump (agglutinate) and leading to complement-mediated hemolysis. Measurement of cold agglutinin titers is used when cold agglutinin disease, a form of autoimmune hemolytic anemia, is suspected. Because IgM is very efficient at fixing complement, even mild cooling of peripheral blood (fingers, toes, ears) can trigger hemolysis in affected individuals.

Associated Diseases


Cold agglutinins are seen in several conditions, including both malignancies and infections:

• Lymphoid malignancies:
  • Lymphoma
  • Chronic lymphocytic leukemia (CLL)
  • Waldenström macroglobulinemia


• Infectious causes:
  • Mycoplasma pneumoniae infection (classic association)
  • Infectious mononucleosis (EBV)

These disorders trigger the production of IgM antibodies that bind RBCs at lower temperatures, leading to hemolysis.


When to Choose Cold Agglutinins as the Correct Answer

You should select cold agglutinins when a patient presents with:

• Signs of hemolytic anemia (fatigue, jaundice, dark urine)
• Symptoms worsened by cold exposure, such as:
  • Painful, blue or purple discoloration of fingers, toes, ears, or nose
  • Livedo reticularis–like changes
  • Acrocyanosis​

This cold-induced discoloration reflects peripheral agglutination of RBCs and complement activation.


Important distinction:
Do not confuse cold agglutinins with cryoglobulins.

• Cold agglutinins → cause hemolysis and cold-induced skin color changes.
• Cryoglobulins → cause purpuric skin lesions, arthralgias, and glomerulonephritis, not hemolysis.

Diagnostic Tests: Decay-Accelerating Factor (DAF) / CD55–CD59 Assay

Overview

The CD55/CD59 assay—also known as the decay-accelerating factor (DAF) assay—is a laboratory test used to evaluate the presence of GPI-anchored protective proteins on the surface of red blood cells. In paroxysmal nocturnal hemoglobinuria (PNH), a mutation in the PIGA gene prevents the attachment of these proteins, particularly CD55 and CD59, leaving RBCs vulnerable to complement-mediated lysis. Measuring the presence or absence of CD55/CD59 via flow cytometry allows clinicians to directly identify PNH cells.

When This Is the Most Accurate Diagnostic Test

The CD55/CD59 flow cytometry assay is the most accurate and definitive test for diagnosing paroxysmal nocturnal hemoglobinuria. Unlike older screening tests such as the sucrose lysis test or Ham’s test, flow cytometry precisely quantifies the proportion of blood cells lacking complement-regulating proteins. Because it identifies the exact molecular defect, it remains the gold standard for diagnosing PNH.


When to Choose DAF (CD55/CD59) as the Correct Answer

You should select the CD55/CD59 assay when a clinical presentation strongly suggests PNH, especially in cases involving:

• Intermittent dark urine, often most noticeable in the morning due to overnight complement activation
• Pancytopenia, reflecting bone marrow failure that often accompanies PNH
• Unexplained large-vessel thrombosis, particularly in unusual locations such as hepatic, portal, or cerebral veins
• Hemolysis with a negative Coombs test, consistent with complement-mediated destruction rather than autoimmune hemolysis

Whenever these features cluster together, particularly dark morning urine and thrombosis, the DAF (CD55/CD59) assay is the diagnostic test of choice.

Diagnostic Tests: Burr Cells​

Overview — What Burr Cells Are

Burr cells, also known as echinocytes, are red blood cells that display numerous short, blunt, and regularly spaced spicules on their surface. Their uniform projections give them a spiky appearance, but unlike spur cells, these projections are consistent and evenly distributed. Burr cells typically reflect alterations in the red cell membrane caused by metabolic abnormalities.


Conditions Associated With Burr Cells

Burr cells appear most commonly in systemic diseases that affect the biochemical environment or membrane composition of red blood cells. They are classically associated with:

• End-stage renal disease (ESRD)
  Uremic toxins disrupt red cell membranes, producing echinocyte formation.
• Liver disease
  Changes in plasma lipid composition and membrane stability can lead to burr cell development.

While some burr cells may arise from artifact during slide preparation, their presence in an appropriate clinical scenario strongly supports renal or hepatic pathology.


Differentiating Burr Cells From Spur Cells

It is important to distinguish burr cells (echinocytes) from spur cells (acanthocytes) because they indicate different underlying conditions.

• Burr cells (echinocytes):
  • Many short, uniform, regularly spaced spicules
  • Associated with ESRD and liver disease


• Spur cells (acanthocytes):
  • Few, irregular, unevenly spaced spicules
  • Seen in severe liver disease, some hereditary spherocytosis, and myelodysplastic syndromes​

The key distinction is uniformity: burr cells have regular projections, while spur cells have irregular, asymmetric ones.

Diagnostics Tests: Russell’s Viper Venom Clotting Time (RVVT)
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Overview — What RVVT Is

Russell’s Viper Venom Clotting Time (RVVT) is a phospholipid-dependent coagulation test used to evaluate abnormalities in the clotting cascade.
The venom directly activates factor X, bypassing most of the intrinsic pathway. Because the test depends on phospholipids, it becomes prolonged when antiphospholipid antibodies interfere with the reaction.


A specialized form, the dilute Russell viper venom test (dRVVT), is used to increase sensitivity for lupus anticoagulant detection.


When RVVT Is Indicated

RVVT is used primarily in the detection of antiphospholipid antibodies, especially the lupus anticoagulant.
You should think of this test when:

• A patient has a prolonged PTT
• The PTT does not correct with a mixing study (suggesting an inhibitor rather than a factor deficiency)
• You need confirmation that the inhibitor is specifically a lupus anticoagulant

RVVT is more specific than traditional PTT testing for diagnosing lupus anticoagulant.


Common Clinical Scenario

A typical clinical vignette that points toward lupus anticoagulant and therefore RVVT includes:

• A woman, often of childbearing age
• With or without known systemic lupus erythematosus (SLE)
• Recurrent spontaneous abortions (classic high-yield clue)
• History of unexplained venous or arterial thrombosis, such as:
  • Deep vein thrombosis
  • Pulmonary embolism
  • Stroke in a young patient


• Or delivering an infant with congenital heart block (neonatal lupus can occur from anti-Ro/SSA, often coexisting with antiphospholipid antibodies)

These scenarios reflect the prothrombotic nature of antiphospholipid syndrome despite paradoxically prolonged PTT.
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Diagnostic Tests: Ristocetin Cofactor Assay​

Overview — What This Test Is

The Ristocetin Cofactor Assay is a laboratory test that evaluates the functional activity of von Willebrand factor (VWF).
Ristocetin acts as a surrogate for the endothelial surface, promoting VWF-mediated platelet agglutination. When VWF is present and functioning normally, platelets will clump in response to ristocetin.


This assay, together with a VWF antigen level, is considered one of the most accurate tests for diagnosing von Willebrand disease (vWD).


When This Is the Correct Answer

Choose the Ristocetin Cofactor Assay when a patient presents with:

• Platelet-type bleeding, such as:
  • Epistaxis
  • Petechiae
  • Purpura
  • Gum bleeding or gingival bleeding


• Normal platelet count
• Normal VWF level, but clinical suspicion for vWD remains
  (e.g., VWF is present but non-functional)

This pattern suggests qualitative VWF defects, particularly Type 2 von Willebrand disease, where VWF quantity may be normal but function is impaired.


The ristocetin assay detects impaired VWF-mediated platelet adhesion, helping distinguish between quantitative and qualitative defects.

Diagnostic Tests: Anti–Intrinsic Factor Antibodies & Antiparietal Cell Antibodies
Overview

Anti–intrinsic factor (anti-IF) antibodies and antiparietal cell antibodies are highly specific markers used to confirm the diagnosis of pernicious anemia, an autoimmune cause of vitamin B₁₂ deficiency. In pernicious anemia, the immune system attacks either intrinsic factor itself or the gastric parietal cells that produce intrinsic factor and gastric acid. Because vitamin B₁₂ cannot be absorbed without intrinsic factor, autoimmune destruction leads to severe B₁₂ deficiency over time. Anti-intrinsic factor antibodies are nearly 100% specific for pernicious anemia, while antiparietal cell antibodies support the diagnosis but are less specific, as they can also appear in autoimmune gastritis.

When These Antibody Tests Are Used

These antibody tests are ordered after confirming vitamin B₁₂ deficiency, to determine its underlying cause. You should select anti-IF and antiparietal cell antibody testing when evaluating a patient with confirmed B₁₂ deficiency—either through low serum B₁₂ levels or elevated methylmalonic acid—and you need to clarify whether the etiology is pernicious anemia. The clinical scenario usually includes macrocytic anemia, neurologic symptoms, glossitis, or long-standing nutritional deficiency concerns. These antibodies are not used to diagnose B₁₂ deficiency itself, but rather to establish why the deficiency occurred when pernicious anemia is suspected.

Diagnostic Tests: Serum Protein Electrophoresis (SPEP)

Overview — What SPEP Is?

Serum protein electrophoresis (SPEP) is a laboratory test that separates and quantifies the major protein components of the blood. By applying an electric current to serum, proteins migrate based on their size and charge, producing a pattern that distinguishes:

• Albumin
• α1, α2, β, and γ globulin fractions
• Immunoglobulins (IgG, IgA, IgM, etc.)

This allows clinicians to identify abnormal elevations—especially monoclonal spikes (“M-spikes”) that suggest plasma cell disorders.

When SPEP Is the Correct Answer

Choose SPEP when evaluating:

• Elevated total serum protein
• Unexplained anemia, especially with rouleaux formation
• Bone pain or lytic lesions on X-ray
• Renal failure of unclear origin
• Symptoms suggestive of plasma cell dyscrasias (fatigue, weight loss, recurrent infections)

SPEP is the best initial test to diagnose multiple myeloma, particularly when:

• X-rays for lytic lesions have already been performed
• Or when imaging is not an available answer choice

It can also help evaluate for Waldenström macroglobulinemia and other gammopathies.

Most Common Reason for an Abnormal SPEP
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The most common cause of an abnormal SPEP—particularly a monoclonal spike—is:

Monoclonal Gammopathy of Unknown/Undetermined Significance (MGUS)

MGUS is far more common than multiple myeloma and is often asymptomatic, although it requires monitoring due to its risk of progression.


Most Accurate Diagnostic Test

For confirming an IgG monoclonal abnormality detected on SPEP, the most accurate test is:

Bone Marrow Biopsy

Findings diagnostic of multiple myeloma include:

• >10% clonal plasma cells
• Evidence of organ damage (CRAB criteria: hyperCalcemia, Renal failure, Anemia, Bone lesions)

For Waldenström macroglobulinemia, SPEP will show an IgM spike, and marrow biopsy will show lymphoplasmacytic cells.

If you’d like, I can produce a quick comparison table: SPEP vs. UPEP vs. Immunofixation, or a high-yield myeloma diagnostic algorithm.