- Confirm the sample identity, location, date and reason before interpreting numbers.
- Read the result together with its unit, qualifier, reporting limit and any prescribed or reference value.
- Separate microbiological, chemical and indicator parameters; they do not all mean the same thing.
- Keep the original laboratory report and link any investigation, corrective action and repeat sample back to it.
1. Check the sample identity first
Before reading any flagged value, confirm that you are looking at the correct supply and the correct sampling event. Check the property or supply name, laboratory reference, sampling point, date and time, sampler, and the stated reason for the sample where those details are shown.
The sampling point matters. A kitchen tap, treatment outlet, storage-tank outlet and raw-water source can legitimately produce different results. A result that is alarming at the consumer tap may have a very different interpretation if it actually came from untreated source water. Keep the laboratory report linked to a clear sampling-point name that also appears on your schematic or monitoring record.
2. Understand the report columns
Laboratory formats vary, but most reports contain some combination of the parameter name, result, unit, method, accreditation status or method reference, a reporting or detection limit, and a comparison value or flag. Some also include comments from the laboratory.
Read across the whole row. A number by itself is not enough. For example, “12” is meaningless until you know whether it is 12 mg/L, 12 µg/L, 12 colony-forming units, 12 NTU or something else entirely.
| Report field | What to check |
|---|---|
| Parameter | Exact analyte or microbiological organism being reported. |
| Result | The measured value, count or reported status. |
| Unit | Never compare a result with a standard until the units match. |
| Qualifier | Symbols such as <, > or laboratory-specific codes can change the meaning of the result. |
| Reference / prescribed value | Check what the laboratory is comparing the result against and whether it applies to that sample. |
| Comments | Laboratory notes may explain detection limits, uncertainty, method issues or interpretation boundaries. |
3. Read units before comparing numbers
Common chemical units include milligrams per litre (mg/L) and micrograms per litre (µg/L). One milligram is 1,000 micrograms, so copying a number without its unit can create a thousand-fold error. Microbiology may be reported as counts per defined sample volume, while turbidity is commonly expressed in NTU and pH is a logarithmic measure rather than a concentration.
Do not convert units mentally when the result matters. Use the exact unit printed by the laboratory and compare it only with a value expressed in the same unit. If the report and regulatory value appear to use different units, ask the laboratory or local authority to confirm the comparison.
4. Qualifiers, detection limits and symbols
A less-than sign before a result usually means the concentration was below the laboratory's stated reporting or detection threshold. It does not necessarily mean the substance was absent or that the true value was zero. A greater-than sign can mean the result exceeded the analytical range used for that test. Laboratories may also use letters, asterisks or footnotes to identify estimated results, subcontracted tests or method limitations.
Always read the report key and footnotes. Do not copy “<0.5” into a workspace as “0” unless the laboratory has specifically told you that is the correct way to handle it for your purpose. Preserve the original text value where possible.
5. Regulatory values, limits and flags
For private water supplies in England, the Regulations contain prescribed concentrations and values for relevant parameters, while some parameters are indicators used to show changes in water quality, treatment performance or acceptability. A laboratory may highlight results outside a reference value, but the colour or symbol on a commercial report is not itself the legal decision.
The local authority considers the result in the context of the supply, the Regulations, the risk assessment and any investigation. A result can also matter operationally even when it is not presented as a simple pass/fail threshold — for example, a change in turbidity may indicate a treatment or source problem and can affect disinfection performance.
6. Microbiological parameters
Microbiological results deserve prompt attention because they can indicate faecal contamination, ingress, poor hygienic conditions or treatment failure. DWI identifies organisms such as E. coli and enterococci as associated with sewage, sewage effluent or animal waste, while other microbiological parameters can provide additional information about the condition of the supply.
Do not treat a microbiological result as merely a number to trend. If a report shows a microbiological failure or an unexpected detection, follow the laboratory and local-authority route promptly. The correct response depends on the organism, sample point, supply type, treatment, recent events and the authority's assessment of health risk.
7. Chemical parameters
Chemical results can reflect geology, catchment activity, treatment chemicals, plumbing materials or contamination. DWI's contaminant guidance links nitrate with fertilisers, animal waste and sewage effluent; iron and manganese can occur naturally or arise from treatment/media and pipework; lead, copper and nickel can be influenced by plumbing and corrosion; and pesticides can reflect agricultural or other catchment use.
A chemical result therefore needs source-to-tap context. Do not jump straight from “high iron” to buying an iron-removal unit, or from a metal result to blaming the borehole. Check where the sample was taken, historic results, source characteristics, plumbing materials, treatment condition and the risk assessment.
8. Indicator and operational parameters
Parameters such as turbidity, colour, conductivity, pH, taste and odour can provide useful evidence about the condition of the source, treatment and distribution system. DWI notes that turbidity can rise after heavy rainfall and can reduce the effectiveness of disinfection. Changes in these parameters can therefore be operationally important even when the immediate question is not a direct toxicological one.
Look for abnormal change as well as absolute values. A stable supply that suddenly becomes more turbid after rainfall, or a treated-water pH that changes after maintenance, may justify investigation even before a broader pattern emerges.
9. Group A, Group B and sample reason
For Regulation 9 supplies, DWI distinguishes Group A and Group B monitoring. Group A provides routine information about compliance, organoleptic and microbiological quality and treatment effectiveness; Group B covers the wider Schedule 1 parameters not already handled as Group A, with additional parameters added where the risk assessment requires them.
The reason for the sample should stay with the report. A result taken for Group A, Group B, additional risk-assessment monitoring or an investigation after a failure has different context. Do not merge all laboratory reports into one undifferentiated list.
10. Read patterns, not isolated numbers
One result can be decisive, particularly where it indicates a serious microbiological or chemical problem, but a sequence of comparable results can reveal information that a single row cannot. Trends may show deterioration after heavy rainfall, seasonal source changes, filter breakthrough, corrosion patterns or improvement following treatment work.
Only compare like with like. Use the same parameter, compatible method and units, and a comparable sampling point. A raw-water result should not be plotted against a treated kitchen-tap result as though they are equivalent. Note treatment changes, maintenance, weather events and sampling-point changes beside the trend.
11. What to do with an abnormal result
First preserve the original report and identify exactly what is abnormal. Then record who was informed and what advice was given. Where a regulatory parameter has breached or the result may indicate unsafe water, the local authority has an investigation and enforcement role under the private-water-supply framework; the response is not simply to take another sample and hope for a different number.
Use the dedicated failed-test guide for the investigation, user-protection and corrective-action pathway. Repeat sampling can be important for diagnosis and verification, but DWI has warned that resampling alone does not replace a proper investigation of a regulatory breach.
12. What to record alongside the report
- The original PDF or laboratory document, unchanged.
- Supply name, sampling point, date/time and laboratory reference.
- Reason for the sample: Group A, Group B, risk-based, investigative or operational.
- Who reviewed the result and when.
- Any laboratory, local-authority or professional advice received.
- Investigation notes, treatment readings and site observations relevant to an abnormal result.
- Corrective actions, responsible person and completion evidence.
- Repeat or verification reports linked back to the original event.
The laboratory-results records guide covers how to organise this evidence estate. This page is deliberately about reading the report rather than building the filing system.
13. Common interpretation mistakes
- Reading the number without the unit. This can make a correct result look wildly wrong.
- Turning “<” into zero. Preserve the qualifier and reporting limit.
- Assuming every flag means the same level of health risk. Parameters have different significance and require context.
- Assuming “not flagged” means “nothing to investigate”. Trends and operational changes may still matter.
- Comparing raw and treated sample points. Their purposes are different.
- Ignoring the sampling reason. Routine monitoring and investigative samples are not interchangeable.
- Discarding the original report after entering results elsewhere. Keep the source document.
- Self-clearing a failure after one repeat sample. Follow the authority's investigation and close-out requirements.
14. Practical report-reading checklist
- Confirm supply, sample point, date, sampler and laboratory reference.
- Read each result with its exact unit and qualifier.
- Read the laboratory key, footnotes and method notes.
- Check the comparison value applies to that parameter and sample context.
- Identify whether the parameter is microbiological, chemical or indicator/operational.
- Keep the sample reason visible.
- Compare trends only across genuinely comparable samples.
- Escalate abnormal or potentially unsafe results rather than interpreting them alone.
- Link advice, actions and repeat samples to the original report.
15. Frequently asked questions
What does “<” before a laboratory result mean?
It normally means the result was below the laboratory's stated reporting or detection threshold. It does not necessarily mean the true concentration was zero. Read the laboratory's own notes for the exact meaning.
Can I compare mg/L and µg/L directly?
No. One milligram equals 1,000 micrograms. Convert carefully or ask the laboratory to confirm the comparison before drawing conclusions.
Does a red or failed flag automatically mean the water is unsafe?
Not every parameter has the same health significance, but a flagged regulatory or abnormal result must not be dismissed. Follow the laboratory and local-authority advice for the parameter and supply.
What is the difference between Group A and Group B on a report?
For Regulation 9 supplies, Group A is the routine monitoring group used for core compliance, organoleptic, microbiological and treatment-effectiveness information. Group B covers the wider Schedule 1 parameters not already handled as Group A, with additional risk-based monitoring where required.
Can I rely on one good repeat sample after a failed result?
No. A repeat sample may help verify conditions, but DWI states that simply resampling does not constitute the investigation required after a regulatory breach.
Should I keep the original laboratory PDF if I enter the results into software?
Yes. Keep the original report as source evidence and use software or spreadsheets as an organised working record linked back to that document.
Keep the laboratory result connected to what happened next.
Private Water Supply Manager helps organise laboratory reports, sampling records, corrective actions and follow-up evidence without pretending to decide whether water is safe.
- DWI: What sampling is required?
- DWI: Common contaminants and their sources
- DWI: Manual on treatment for small water supply systems
- DWI: Local-authority FAQs and action after a breach
This guide is England-focused. Laboratory formats and qualifiers vary; use the current report notes and local-authority advice for the actual sample.