Cell Washer

The automated cell washer is a specialized serological centrifuge designed for a single, critical purpose: to standardize the washing phase of the Antiglobulin Test (Coombs Test). While manual washing is possible, the cell washer is preferred in high-volume laboratories to reduce technical time and, more importantly, to eliminate the variable of human error in the most sensitive step of pre-transfusion testing. Its primary function is to remove unbound globulins (plasma proteins) from the test system while retaining the red blood cells, ensuring that the Anti-Human Globulin (AHG) reagent reacts only with antibodies attached to the cells

Principles of Operation

The cell washer is essentially a centrifuge integrated with a hydraulic pump and a decanting mechanism. It automates a sequence of adding saline, spinning, decanting, and agitating

The Wash Cycle Sequence

A standard cycle consists of the following automated steps, which are typically repeated 3 or 4 times:

1. Fill: A precise volume of physiological saline (\(0.85\text{-}0.90\%\)) is injected into each tube. The force of the stream helps resuspend the cell button
2. Spin (Centrifugation): The rotor accelerates to pack the red blood cells at the bottom of the tube. This spin is usually shorter/faster than a standard serological spin to minimize time
3. Decant: The rotor slows, and the centrifuge bowl (or a catch basin) utilizes centrifugal force to eject the supernatant (saline + unbound proteins) over the top of the tubes into a waste reservoir. The red cell button remains at the bottom due to the angle and force
4. Agitate (Resuspension): Before the next fill, the rotor may shake or vibrate, or rely on the next saline injection to break up the button for thorough washing

The Decanting Mechanism

This is the distinct feature of a cell washer

• Manual Washing: Relies on the laboratory scientist inverting the tube and “flicking” the wrist. This varies by strength and technique
• Automated Decanting: Relies on geometry and physics. The saline level rises during the fill. During the decant step, the specific RPM and rotor angle drive the liquid up the tube walls and out, while the heavier RBCs remain packed in the bottom corner. This ensures a consistent volume of residual saline is left (the “dry button”)

Clinical Significance: Why Automation Matters

The wash step is the most common source of error in the Indirect Antiglobulin Test (IAT)

• Neutralization of AHG: AHG (Coombs reagent) is highly sensitive to free human serum globulins. Even a microscopic amount of residual plasma left in the tube can bind to the AHG, neutralizing it before it finds the antibodies on the red cells
  • Result: False Negative (The neutralized AHG cannot crosslink the cells)
• Standardization: An automated washer adds the exact same amount of saline and spins for the exact same time for every tube, every time. This reproducibility is vital for QC and Proficiency Testing

Quality Control & Calibration

Because the washer performs multiple mechanical tasks, its QC is more complex than a standard centrifuge

Saline Fill Volume

• The Check: Verify that the instrument delivers a consistent volume of saline to all 12 (or 24) positions
• Procedure: Run a “Fill” cycle with empty tubes. Measure the volume in each tube using a graduate or pipette
• Standard: The volume should be uniform (e.g., all tubes within \(\pm 10\%\) of the target) and sufficient to fill the tube (~75-80% full) without overflowing. Low fill volume leads to inadequate washing (False Negative)

Decant Efficiency (Residual Saline)

• The Check: Verify that the instrument removes the saline effectively, leaving a “dry” cell button
• Procedure: Weigh a set of tubes with a known cell volume before and after a wash cycle
• Standard: There should be minimal residual saline. If too much saline is left, it dilutes the AHG reagent added later, weakening the reaction

Speed (RPM) & Timer

Just like a standard centrifuge, the RPM and timer must be calibrated quarterly to ensure the cells are packed hard enough to stay in the tube during the decant step but not so hard they cannot be resuspended

Daily QC: The Coombs Control (Check Cells)

The ultimate daily QC for the cell washer is the Check Cell step in routine testing

• If the washer is failing (e.g., clogged saline lines, poor decanting), the AHG will be neutralized
• Result: All negative tests will fail to agglutinate with Check Cells
• Action: If Check Cells fail, the washer is the prime suspect. Check saline levels, look for crimped tubing, or clean the distributor head

Troubleshooting Common Problems

• Problem: “My cell buttons are disappearing.” (Low Cell Recovery)
  • Cause: The spin speed is too low (cells aren’t packed tight enough before decant) or the saline fill pressure is too high (blasting cells out of the tube)
  • Impact: Loss of patient sample; potential false negative due to lack of antigen
• Problem: “The Check Cells are consistently weak or negative.”
  • Cause: Incomplete decanting (supernatant remaining neutralizes AHG) or insufficient saline fill (dilution ratio too low)
  • Impact: False Negative IAT results (Patient safety risk)
• Problem: “One specific tube position is always dry/empty.”
  • Cause: A clogged port in the distributor head. Saline crystallizes easily and blocks the small needles
  • Maintenance: The distributor head must be removed and soaked in warm water/vinegar periodically to dissolve salt crystals
• Problem: “The centrifuge is vibrating violently.”
  • Cause: Imbalance. Even though it fills automatically, if one port is clogged, that tube remains empty while others fill, creating a severe weight imbalance during the spin