Aflatoxin M1 in Milk: ELISA Screen, HPLC Confirmation and a Failing z-Score
On 12 March a Food Safety Officer draws a composite milk sample at a cooperative chilling centre in Anand and splits it into four sealed parts. The part sent for analysis reaches the district laboratory 22 hours after collection in a 500 mL PET bottle with no preservative, having travelled at 28-34 degrees C. The laboratory screens for aflatoxin M1 by ELISA and reports 0.62 micrograms/kg; the limit in the FSS (Contaminants, Toxins and Residues) Regulations is 0.5 micrograms/kg. On the Food Analyst's direction the sample is re-tested by HPLC with fluorescence detection and gives 0.54 micrograms/kg. The laboratory's scope of NABL accreditation does not include aflatoxin M1 in milk; its expanded uncertainty at this level is 0.09 micrograms/kg (k = 2); its most recent proficiency testing round for this analyte returned a z-score of 3.4; and the analyst who signed the report joined three weeks ago with no training record for mycotoxin testing. The seals were intact on receipt. The Food Analyst must decide whether to certify the sample as non-compliant.
1. Should the sample be certified as non-compliant? Apply a decision rule to the result and the uncertainty.
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The reportable result is the confirmatory one. ELISA is a screening technique; a positive screen must be confirmed by HPLC-FLD or LC-MS/MS, and the HPLC value of 0.54 micrograms/kg supersedes 0.62.
Apply the uncertainty. The limit is a legal maximum. Where exceeding it triggers a finding against the FBO, the finding should stand only if the result exceeds the limit after allowing for measurement uncertainty at the stated confidence level. Here 0.54 minus the expanded uncertainty of 0.09 gives 0.45 micrograms/kg, which is below 0.5. The result therefore does not establish, at 95% confidence, that the true value is above the limit.
Conclusion. Certify the sample as 'does not exceed the prescribed limit' - not as non-compliant - and record the measured value, the method, the uncertainty and the decision rule in the report. Do not report 'compliant' as though the milk were clean: 0.45 to 0.63 is a band that straddles the limit, so the honest statement is that exceedance is not proven. The same arithmetic should be applied consistently in every case, which means the laboratory must have a written decision rule before results are issued, not after the FBO objects.
2. Which laboratory quality failures undermine the evidentiary value of this report, and how serious is each?
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Method not in the accredited scope. Accreditation is the assurance that the laboratory can perform this analyte competently. Testing outside scope is not automatically unlawful, but the report loses the presumption of reliability and the prosecution will have to lead evidence of validation. This is a serious defect.
z-score of 3.4. A score beyond |3| is 'unsatisfactory' - the laboratory's result is an outlier against the assigned value. It demands root-cause investigation, corrective action and suspension of the method until performance is demonstrated, not merely a note on file.
Untrained analyst. No training record for mycotoxin testing means competence is unevidenced; the signature of a person not recognised for that analysis weakens the report.
Sample handling. An unpreserved milk sample held at 28-34 degrees C for 22 hours and analysed on day three will have undergone microbial growth, acidification and fat separation. Aflatoxin M1 is relatively heat stable, but matrix changes affect extraction recovery and immunoassay binding, and the sample is no longer representative of the milk in the Bulk Milk Cooler.
Missing validation data. Recovery, LOD/LOQ and linearity must be documented for the matrix. Taken together, these defects mean the report would be vulnerable even if the number had exceeded the limit with uncertainty.
3. What should the Food Analyst do procedurally, and what should be fixed before the next such sample?
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Procedural steps. Issue the report in the prescribed form within the prescribed time, stating the method, the result, the uncertainty and the decision rule; do not certify non-compliance. If non-compliance is suspected on other grounds, the correct route is a fresh sample drawn and transported under proper conditions - chilled, with a preservative where prescribed, delivered to a laboratory with the analyte in its accredited scope - rather than a second attempt on a degraded sample. The FBO must be given the report and its right to have the sample analysed at a referral laboratory, and the chain of custody, seal numbers and receipt time must be recorded on the file.
Laboratory fixes. Bring aflatoxin M1 in milk into the NABL scope after validation; investigate the PT outlier and document corrective action; restore satisfactory PT performance before resuming routine reporting; train and authorise named analysts with records; write and enforce a sample acceptance policy that rejects samples outside the temperature and time limits, with a non-conformance note; and establish a documented decision rule with uncertainty for every quantitative limit. The Food Analyst should also verify that the sampling officer is using the correct bottle, preservative and transport protocol, because an otherwise sound laboratory can be defeated at the point of collection.