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Diagnostics2026-05-22 · 13 min read

Hematology Analyzer QC Workflow for Vet Clinics: Flags, Blood Smears, Calibration

Build a hematology analyzer QC workflow for a veterinary practice: analyzer flags, blood smear review triggers, ASVCP quality-control guidelines, calibration, maintenance, and when to send samples.

Ran Chen
Ran Chen
Founder, VetMedGuide. Life-sciences operator and 10× global market-access lead.
Published

A veterinary practice that runs an in-house hematology analyzer without a structured quality-control (QC) program is making clinical decisions on numbers it has not verified. The American Society for Veterinary Clinical Pathology (ASVCP) Guidelines for Quality Assurance and Standards for Veterinary Clinical Pathology state that laboratories — including in-clinic point-of-care laboratories — must have documented QC procedures covering instrument calibration, control-material analysis, blood smear review, repeat criteria, and medical review of results. The ASVCP's Point-of-Care Testing (POCT) QA Guidelines reinforce that "use of written policies, standard operating procedures, forms, and logs" and "assessment of instrument analytical performance and use of both statistical quality control and external quality assessment programs" are minimum standards.

This article provides a practical QC workflow for a veterinary practice operating an in-house hematology analyzer. It covers what analyzer flags mean and how to respond, when a blood smear must be reviewed, the ASVCP QC framework adapted for general practice, maintenance schedules, and the decision criteria for sending samples to a reference laboratory instead.

Why hematology QC fails in general practice

Three dynamics undermine hematology QC in veterinary clinics:

  1. The analyzer seems to work, so no one checks it. Impedance-based and laser-flow cytometry analyzers produce numbers on every run. When those numbers are plausible, the team trusts them without verification. But plausible does not mean accurate. A systematic bias — for example, a consistently low hematocrit due to calibration drift — can go undetected for months if no control material is run.

  2. Quality control materials are expensive and short-dated. Commercial fixed-cell QC materials (e.g., IDEXX e-CHECK, Sysmex e-CHECK) have limited shelf lives and must be stored and handled precisely. A 2022 study in Veterinary Clinical Pathology found that "the short shelf life of the FC-QCM creates a significant financial and logistical burden on veterinary clinics and can prevent clinics from realizing the full benefit" of their QC program. Clinics run QC sporadically or skip it entirely.

  3. No one reviews blood smears. Blood smear review is the single most important manual QC step available in any veterinary practice. The ASVCP guidelines list blood smear review as a required QC element for hematology testing. Yet many practices rely entirely on the analyzer's numeric output and never look at the slide. The Royal Canin Academy hematology FAQ summarizes the problem: "For proper interpretation of hematology data in the veterinary clinic, the results must reflect the patient's real condition." Blood smear review is the check that confirms they do.

Analyzer flags: what they mean and how to respond

Most veterinary hematology analyzers generate flags when the instrument encounters cells or patterns it cannot classify with confidence. Understanding these flags is the first line of defense against erroneous results.

Common flag types

Flag type What it indicates Required action
Suspect WBC flags (blasts, left shift, atypical lymphocytes, abnormal lymphocytes/blasts) Abnormal white blood cell populations that the analyzer cannot reliably classify Blood smear review with manual differential
WBC unreliability flag Instrument cannot produce a reliable WBC count or differential Check sample for clots; rerun; if flag persists, review smear and consider send-out
Suspect RBC flags (NRBCs, RBC agglutination, turbidity/HGB interference, iron deficiency, fragments) Abnormal erythrocyte features that affect accuracy of RBC, HGB, or HCT values Blood smear review; verify HGB against spun PCV
Platelet clump flag Platelet aggregation leading to falsely low platelet count Check blood tube for visible clots; review smear for clumps; estimate platelet count from smear
Hash mark (--.--) replacing a numeric result Analyzer unable to return any result for the parameter Blood smear review; send to reference lab if smear does not resolve the question
Asterisk (*) next to a result Analyzer questions the accuracy of that specific population Blood smear review to verify or correct

The IDEXX ProCyte Dx Operator's Guide defines these flags explicitly: "The flagging for the ProCyte Dx analyzer signals the user that an abnormal cell or group of cells is present and it cannot be characterized in the normal hemogram." The Heska HemaTrue product manual similarly states: "Abnormal test results, including both results with sample pathology messages and results outside the normal range, should be evaluated by microscopic examination of the blood film."

When flags are absent but results are still wrong

Flags do not catch every error. Common situations where results are inaccurate despite the absence of flags include:

  • Platelet count in cats. Feline platelets clump readily during venipuncture and sample handling. The analyzer may report a normal platelet count while clumps are visible on the smear. The ASVCP Allowable Total Error guidelines for hematology note that "blood specimens from cats are known to clot quickly, and their platelets are frequently clumped upon evaluation of a blood smear."
  • Hemolysis and lipemia. Hemolyzed samples may have falsely elevated hemoglobin and MCHC. Lipemic samples can interfere with multiple parameters. These preanalytical errors do not always trigger instrument flags.
  • Old or improperly mixed samples. EDTA blood that has sat too long or was not adequately inverted after collection develops in vitro changes that the analyzer reports as pathology.

Blood smear review: the essential manual QC step

Blood smear review serves two functions: it verifies the analyzer's numeric output, and it provides morphological information the analyzer cannot.

When to review a smear

Every practice should establish written criteria for mandatory blood smear review. The International Society for Laboratory Hematology (ISLH) consensus rules, adapted for veterinary use and supported by the ASVCP guidelines, provide a framework:

Trigger Action
Any suspect flag (blasts, left shift, atypical lymphocytes, NRBCs, fragments, agglutination) Full smear review with manual differential
WBC count < 4,000/μL or > 30,000/μL Smear review
Platelet count < 100,000/μL Smear review to rule out clumping; estimate count from smear
Hemoglobin < 7 g/dL Smear review; verify with spun PCV
MCV outside reference interval Smear review for abnormal RBC morphology
Neutrophil count < 1,000/μL Smear review for left shift, toxic change
New patient with unexplained clinical signs Smear review regardless of flag status
Discrepancy between two consecutive results in the same patient Smear review to identify cause

What to look for on the smear

  1. Verify cell counts. Estimate the WBC and platelet counts from the smear and compare with analyzer values. A properly prepared smear should show roughly 10–30 WBCs per 40× field in a normal patient. Platelet estimates should be approximately 10–20 platelets per 100× oil-immersion field for a normal count.

  2. Perform a manual differential. Count 100 white blood cells and classify them. Compare the percentages and absolute counts with the analyzer's differential. If the manual differential differs significantly, trust the smear — the analyzer may be misclassifying cells.

  3. Evaluate erythrocyte morphology. Look for anisocytosis, poikilocytosis, polychromasia, spherocytes, acanthocytes, schistocytes, and Heinz bodies. These findings change the diagnostic interpretation and are invisible to the analyzer's numeric output.

  4. Check for platelet clumps. Scan the feathered edge and body of the smear. Platelet clumps invalidate the analyzer's platelet count. Report an estimated count from the smear instead.

  5. Look for unexpected cells. Circulating blast cells, mast cells, or organisms (hemoparasites) may not trigger a flag on every analyzer but have major diagnostic implications.

The ASVCP QC framework adapted for general practice

The ASVCP provides the authoritative QC framework for veterinary laboratories. The key elements adapted for an in-clinic hematology analyzer are:

1. Statistical quality control with control materials

Run a commercial QC material (e.g., IDEXX e-CHECK, Sysmex e-CHECK) at regular intervals. The ASVCP POCT guidelines recommend using "a validated 1-3s control rule for interpretation of control data" where possible given instrument analytical performance. Under this rule, a QC run is flagged for investigation if any single measurand falls more than 3 standard deviations from the target mean.

Minimum frequency:

  • Run QC material at least monthly, or whenever a new lot of reagents is opened.
  • Run QC after any analyzer service event, repair, or relocation.
  • Run QC when results seem inconsistent with clinical findings.

Lot-to-lot comparison: When transitioning to a new lot of QC material, run both the old and new lots side by side. Compare the results to detect lot-to-lot bias. The Veterinary Clinical Pathology study on improving QC for in-clinic hematology analyzers notes that "skipping this step diminishes the ability of the FC-QCM to assess analyzer function and drift over time."

2. Patient-based QC

Not every practice can afford to run commercial QC material every day. Patient-based QC uses the patient samples themselves as a running quality check:

  • Spun PCV vs. analyzer HCT. The ASVCP guidelines state that "spun hematocrits (PCV) should approximate hematocrit (Hct) calculated by an automated analyzer." Set a maximum acceptable difference (commonly ±3% for dogs and cats). If the difference exceeds the threshold, investigate.
  • MCHC as an internal check. An MCHC above the reference interval, in the absence of hemolysis, lipemia, or Heinz bodies, suggests instrument error rather than a true patient abnormality. The ASVCP guidelines note: "In the absence of these conditions, a high MCHC may indicate instrument error."
  • Ret iculocyte correlation. Automated reticulocyte counts should correlate with the proportion of polychromatophilic RBCs seen on the stained smear.
  • Delta checks. For patients with serial CBCs, compare current results with prior values. A large unexpected change between consecutive samples (e.g., HGB drops 4 g/dL in 24 hours without clinical explanation) should trigger investigation.

3. External quality assessment

The ASVCP guidelines recommend participation in an external quality assessment program (proficiency testing) when available. Several veterinary reference laboratories and professional organizations offer EQA programs that send unknown samples to participating clinics for analysis and comparison against peer-group performance.

Participation is not practical for every small clinic, but the principle can be approximated: periodically send duplicate samples to both the in-house analyzer and a reference laboratory. Compare the results. Consistent discrepancies indicate a systematic bias that needs correction.

Maintenance schedule

A hematology analyzer that is not maintained produces unreliable results regardless of how carefully QC is run. The Chemtronics maintenance guidelines for hematology analyzers outline a tiered schedule:

Frequency Task
Daily Surface cleaning; verify aspiration probe is not clogged; check reagent levels; run auto-cleaning cycle if programmed
Weekly Manual probe cleaning; inspect sample pathway for residue; verify waste container is not full
Monthly Run QC material; deep-clean the sample aspiration path; check for crystal buildup in fluidic channels
Semi-annual Service by a qualified technician: inspect valves, chambers, connectors; clean or replace internal fluidic lines; verify calibration against manufacturer standards
As needed Replace reagents per manufacturer lot dating; recalibrate after service events

For cartridge-based or maintenance-free analyzer designs (systems that use single-use staining kits rather than bulk liquid reagents), daily and weekly cleaning tasks are minimized, but QC testing and semi-annual calibration verification remain necessary.

When to send out to a reference lab

In-house hematology is appropriate for most wellness screening, pre-anesthetic evaluation, and monitoring of known conditions. But certain situations require reference-laboratory analysis:

Situation Why send out
Suspect hematologic neoplasia (blasts on smear, unexplained leukocytosis with left shift) Reference lab pathologists can perform manual differentials, cytochemistry, and flow cytometry that are not available in-clinic
Persistent analyzer flags with unresolved findings A board-certified clinical pathologist can provide morphological expertise and diagnostic interpretation beyond what most general practices can achieve
Discrepancy between analyzer results and clinical picture Reference-lab verification resolves whether the issue is the patient, the sample, or the analyzer
Platelet count < 50,000/μL confirmed on smear Severe thrombocytopenia warrants pathologist review and may require additional testing (coagulation panel, tick-borne disease screen, bone marrow aspirate)
Regenerative vs. non-regenerative anemia unclear Reticulocyte count accuracy and interpretation benefit from reference-lab methodology and pathologist correlation
Multi-site practices standardizing results Reference-lab results provide a consistent baseline that in-house analyzers at different locations may not

The decision to send out is not a failure of the in-house system. It is the appropriate escalation when the analyzer's or the practice team's limitations are reached. A practice that sends out 10–20% of its hematology samples and has confidence in the 80–90% it runs in-house has a better QC posture than a practice that runs everything in-house without ever verifying the results.

The QC log: what to record

Maintain a written or digital log that includes:

Field Example
Date 2026-05-22
QC material lot and level e-CHECK (XS) Lot #1234, Level 2
Analyzer ID ProCyte Dx — Serial #PDX-001
Operator J. Martinez, RVT
Results (WBC, RBC, HGB, HCT, PLT) WBC 7.1, RBC 6.8, HGB 14.2, HCT 42%, PLT 298
Target mean for each parameter WBC 7.11, RBC 6.95, HGB 14.5, HCT 43%, PLT 305
Within limits? (1-3s rule) Yes — all parameters within ±3 SD of target
Action taken (if any) None required
Next QC due 2026-06-22

Also log every maintenance event, reagent change, service call, and blood smear review triggered by a flag or abnormal result. The log should be reviewable by the practice manager or medical director at least quarterly.

Sources

Topics
Clinical PathologyClinic Workflow & Operations
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