Knowledge of detecting spurious blood counts is key to effective use of automated cell counters

Dhruv Kamat

Though automated cell counters have become very popular, the availability of validation criteria and reference material is very less. Blind faith in automation should be replaced by an understanding that automated blood counts may be inaccurate, subject to conditions.

Also, the knowledge of detecting spurious blood counts is the key to successful use of automation and so is the subject of this article. It is the responsibility of laboratory staff performing a counter authorising report to detect inaccuracies whenever possible. All suspected spurious cell counts/ morphologies should be validated with microscopic examination of slides. The validation of results generated by automated blood cell counter requires:

(i) Knowledge that an instrument is capable of measuring all variables accurately, has been correctly calibrated and that quality control procedure indicates normal functioning.

(ii) Assessment of each individual count as to whether it is likely to be correct or that, alternatively, it requires further review.

Some conditions that cause spurious counts and recommended checks

Fault in specimen collection or storage

1. Blood from wrong patient blood specimen and request relating to different patient.

2. Specimen diluted (e.g. taken from above intravenous infusion or excess liquid EDTA relative to volume of blood.)

3. Specimen taken in too high concentration of blood.

4. Presence of dust in locally prepared EDTA bottles results in spuriously high cell counts on automated cell counters.

5. Specimen haemoconcentrated due prolonged application of tourniquet.

6. Specimen partly clotted/ lysed /aged blood specimen inadvertently heated or frozen.

Faulty sampling

1. Failure in priming of instrument can cause spurious high/ low counts.

2. Inadequate mixing or specimen of inadequate volume. In general going by laws of fluid dynamics 60 inversions of 2.5 ml blood specimen should be done prior to aspiration on Automated cell counters. (A bulb mixer will be handy.) 3. Aspiration probe uncleaned or blocked by previous sample. The cleanliness of probe can be visually checked in between the runs.

4. Carry-over from preceding very abnormal specimen.(which is minimal with modern instruments employing separate dilution baths for primary and secondary dilutions.)

Faulty Calibration

1. Calibration performed without repeated successive runs of calibrator. Minimum five runs of a calibrator should be performed prior to deciding on calibration factors.

2. Calibration performed under improper instrument conditions or with improper calibrator.

All routine maintenance must be done thoroughly prior to calibration.

Faulty maintenance /other instrument malfunctions/ reagent failure

1. Contamination or deposition in counting fluidics. Deposition in dilution baths and counting fluidics is a common cause of random errors and should be kept an eye upon.

2. Contaminated reagents. It is advised to keep the reagent containers closed and away from dust.

3. Change in conductivity of reagents owing to high temperature conditions.

4. Electrical noise in absence of proper earthling.

5. Mechanical or Hydraulic or Electronic failures inside instrument.

Inaccuracy inherent in specific methodologies

Inaccuracies that are inherent to aperture size, electronic editing, designing methodology of the instruments e.g. Underestimation of MCV by impedance counters in presence of Hypochromia or sometimes linearity problems associated with higher Aperture size instruments.

Inaccuracy due to unusual specimen characteristics

Common cause is erronious HB or red cell indices caused by cold agglutinins or due to unusual hyperlipidaemia or factitious leukocytosis caused by insufficient lysing of NRBCs in specimen characteristics neo-natal samples.