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Hematology

Point-of-Care Analyzers

Point-of-Care Testing (POCT), also known as bedside or near-patient testing, is defined as laboratory testing performed outside of the central laboratory, typically by non-laboratory personnel (nurses, respiratory therapists, or physicians). In Hematology and Coagulation, POCT is utilized when the turnaround time (TAT) of the central lab is too slow for critical clinical decision-making, such as during cardiac surgery, trauma resuscitation, or outpatient anticoagulant monitoring. The laboratory scientist plays a vital role in the oversight, validation, quality control, and regulatory compliance of these decentralized instruments

POCT Hemoglobin & Hematocrit

Measuring hemoglobin and hematocrit at the bedside is critical for assessing acute hemorrhage in trauma or anemia in blood donors. While manual spun hematocrits are the historical standard, handheld photometric devices and conductivity sensors are now the norm

Photometric Hemoglobin (e.g., HemoCue)

The HemoCue system is the standard for POC hemoglobin measurement. It is a modified spectrophotometer that utilizes a disposable microcuvette which serves as both the pipette and the reaction vessel

  • Principle
    • The microcuvette contains dry reagents (typically sodium deoxycholate to lyse RBCs and sodium nitrite/azide) coating the inner walls
    • Blood is drawn into the cuvette via capillary action
    • The reagents convert hemoglobin into Azide-Methemoglobin
    • The instrument measures absorbance at two wavelengths: 570 nm (for hemoglobin) and 880 nm (to correct for turbidity/lipemia)
  • Sources of Error
    • Filling Technique: The most common error. The cuvette must be filled in one continuous motion. Introducing air bubbles creates an optical path with no sample, causing falsely Low results
    • Tissue Fluid: Milking the finger (excessive squeezing) during capillary collection introduces interstitial fluid, diluting the sample and causing falsely Low results

Conductivity Hematocrit (e.g., i-STAT)

Many handheld blood gas analyzers calculate hematocrit based on electrical conductivity rather than direct centrifugation or optical counting

  • Principle: Blood conducts electricity primarily through the plasma (electrolytes). Red Blood Cells act as insulators. Therefore, the conductivity of the whole blood is inversely proportional to the Hematocrit (Packed Cell Volume)
  • The Calculation: The instrument measures conductivity and assumes a normal plasma protein and electrolyte concentration to calculate the Hct. It then calculates Hemoglobin (Hct / 3)
  • Limitations: This method is notoriously inaccurate in patients with abnormal plasma osmolality or improper fluid resuscitation. Saline is highly conductive; if a patient is receiving massive saline infusion, the conductivity increases, leading the instrument to believe there are fewer RBCs, potentially resulting in a falsely low Hematocrit

POCT Coagulation Testing

Coagulation is the sector of Hematology most heavily reliant on POCT, particularly for monitoring high-dose heparin in the operating room (where standard PTT is insufficient) and for Warfarin monitoring in outpatient clinics

Activated Clotting Time (ACT)

The ACT is the standard test for monitoring High-Dose Unfractionated Heparin therapy during cardiopulmonary bypass (CPB), ECMO, and cardiac catheterization. The standard PTT is too sensitive for these massive heparin doses (it would simply read “> 150 seconds” regardless of the dose)

  • Principle
    • Fresh whole blood is added to a cartridge containing a particulate activator (Kaolin, Celite, or Glass Beads)
    • The activator triggers the Intrinsic Pathway (Contact Activation) violently and rapidly
  • Endpoint Detection
    • Mechanical: A plunger or magnet moves through the sample. As the clot forms, the movement is impeded
    • Electrochemical: The conversion of a substrate generates an electrical current upon clotting
  • Clinical Goal: During bypass, surgeons typically aim for an ACT of > 400 seconds to ensure blood does not clot in the bypass machine

PT/INR Monitors (e.g., CoaguChek)

These handheld devices utilize microfluidics and electrochemical biosensors to monitor Warfarin (Coumadin) therapy

  • Principle
    • A drop of capillary blood is applied to a strip containing dried Thromboplastin (Tissue Factor)
    • The clotting reaction (Extrinsic Pathway) begins
    • The thrombin generated cleaves a specific peptide substrate containing an electroactive molecule
    • This cleavage generates a flow of electrons (Amperometric detection). The current generated is proportional to the clotting time
  • Correlation: These devices calculate the INR internally. It is crucial to note that POC INR results may differ from Central Lab Plasma INR results, particularly as the INR increases (> 4.0). Labs must perform periodic correlation studies between the POC device and the main analyzer

Viscoelastic Testing (TEG & ROTEM)

Thromboelastography (TEG) and Rotational Thromboelastometry (ROTEM) are specialized POCT analyzers used in Trauma Centers and Operating Rooms. Unlike standard PT/PTT (which only measure the start of the clot), TEG measures the entire lifecycle of the clot: initiation, strength, stability, and lysis

  • Principle (Cup and Pin)
    • TEG: A sample is placed in a cup that oscillates (rotates back and forth). A pin is suspended in the blood. As the clot forms, fibers connect the cup to the pin, transferring the motion of the cup to the pin. The stronger the clot, the more the pin moves
    • ROTEM: The pin oscillates while the cup remains stationary. As the clot forms, it restricts the movement of the pin
  • Key Parameters
    • R-Time (Reaction): Time to initial fibrin formation (correlates to Coagulation Factors). Treatment: Plasma (FFP).
    • MA (Maximum Amplitude): The widest point of the curve, representing maximum clot strength. This is dependent on Platelet Number and Function and Fibrinogen. Treatment: Platelets.
    • LY30 (Lysis at 30 min): Measures how quickly the clot dissolves. High LY30 indicates Hyperfibrinolysis. Treatment: Tranexamic Acid (TXA).

Platelet Function Analyzers (PFA)

The PFA-100/200 is an automated “near-patient” instrument that replaced the manual Bleeding Time. It screens for qualitative platelet defects (e.g., Von Willebrand Disease) and monitors anti-platelet therapy (Aspirin)

  • Principle (High Shear Flow)
    • Whole blood is aspirated under high vacuum (mimicking high shear stress in capillaries) through a microscopic aperture
    • The aperture membrane is coated with Collagen: and an agonist (either EPI (Epinephrine) or ADP)
    • Platelets adhere to the collagen, activate, and aggregate to plug the hole
  • Endpoint: The Closure Time (CT) - the time required for the platelet plug to completely block the aperture and stop blood flow
  • Interpretation
    • Prolonged COL/EPI + Normal COL/ADP: Indicates Aspirin effect
    • Prolonged COL/EPI + Prolonged COL/ADP: Indicates True Platelet Defect (Glanzmann’s, Bernard-Soulier) or Von Willebrand Disease

Quality & Regulatory Oversight

Because POC testing is often performed by non-laboratorians, the laboratory scientist is responsible for ensuring the system meets CLIA and CAP requirements

  • CLIA Status: Most POC devices are CLIA Waived (simple, low risk of error), but some (like ACT or automated diffs) may be Moderate Complexity. This dictates the level of personnel training required
  • Quality Control (QC)
    • Internal (Electronic) QC: Many devices simulate a signal to verify the electronic components are working. This is performed automatically
    • External (Liquid) QC: Even waived tests require the periodic running of liquid control material (Low and High levels) to verify reagent integrity. This is mandatory for new lot numbers or shipment receipts
  • Connectivity: Modern POCT operations rely on “middleware” that connects the handheld device to the LIS. This ensures results are automatically posted to the patient chart and prevents “unauthorized” operators from performing tests (locking out users who have not completed competency)