What conditions make a manual cell count by hemacytometer (a manual device used for counting cells) preferable to an automated cell count?

When Manual Cell Counting by Hemacytometer is Preferable to Automated Cell Counting

Manual cell counting by hemacytometer is preferable to automated cell counting in all of the above situations (D): when very small volumes of fluid are available, when cell counts are below the linear limits of the instrument, and when very large cells, clumps of cells, crystals, or microorganisms in the fluid can interfere with an automated cell counter.

Reasons for Choosing Manual Cell Counting

1. Small Sample Volumes

• When sample volume is extremely limited (such as with certain body fluids or precious research samples), manual hemacytometer counting allows for analysis of much smaller volumes than automated analyzers require 1
• Automated systems typically require minimum volumes that may not be available in certain clinical scenarios

2. Low Cell Concentrations

• Manual hemacytometer counting remains the gold standard for samples with very low cell concentrations 2
• Automated analyzers have lower limits of detection that may not be adequate for certain specimens:
  • For example, the lower limit of detection for automated systems can be around 30-35 × 10^6/L for RBCs and nucleated cells 2
  • This makes automated systems particularly problematic for cerebrospinal fluid specimens with low cell counts 2
  • Manual counting can detect cell concentrations as low as 5 × 10^3 cells/ml, which is below the functional sensitivity of many automated analyzers 3

3. Interference Factors

• Manual counting is preferred when samples contain elements that can interfere with automated counting, including:
  • Very large cells that may exceed the size parameters of automated analyzers
  • Cell clumps that can clog automated systems or be counted incorrectly
  • Crystals that may be misidentified as cells by automated systems
  • Microorganisms that could be missed or misclassified by automated methods 1, 4
  • Abnormal proteins like cryoglobulins that can precipitate during testing and affect spectrophotometric measurements 1
  • RBC abnormalities such as those in sickle cell disease, which can create cells resistant to standard lysis procedures used in automated analyzers 1

Advantages of Manual Cell Counting in These Situations

• Provides direct visualization of cells and other elements, allowing for better morphological assessment
• Allows the operator to distinguish between cells and non-cellular elements that might confuse automated systems
• Serves as the reference method included in the European Pharmacopeia for cell counting 5
• "Freezes" the culture status, permitting analysis by more than one operator when needed 6

Limitations of Automated Cell Counting

• Automated systems may have difficulty with:
  • Distinguishing between certain cell types and non-cellular elements
  • Accurately counting when there are very few cells present
  • Processing samples with unusual or abnormal cells
  • Analyzing samples with high viscosity or other physical properties that interfere with automated counting 4

Best Practices for Manual Cell Counting

• Use standardized protocols to minimize variability
• Consider automated image mosaicing techniques to improve reliability of manual counts 6
• Ensure proper training of personnel performing manual counts
• When possible, have multiple operators verify counts for critical samples

While automated cell counting offers advantages in terms of speed, efficiency, and standardization for many routine samples, manual hemacytometer counting remains essential in these specific situations where automated methods may fail to provide accurate results.