Dip slides versus laboratory testing: what is the difference?
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Key points
- A dip slide does not test for legionella. It measures the total viable count of general aerobic bacteria. This is the single most misunderstood point in water testing.
- Only laboratory culture to BS EN ISO 11731 (or a qPCR screen) detects legionella specifically and reports a count in cfu/L.
- Dip slides are a legitimate tool in the right place: a cheap, frequent trend check on treated systems such as cooling towers, set into the routine in HSG274 Part 1.
- The two are not rivals for the same job. If the question is “is legionella present?”, only the laboratory answers it. If the question is “is my treatment regime holding?”, the dip slide answers it cheaply.
Dip slides and laboratory culture tests are both sold as water testing, often on adjacent shelves and sometimes on adjacent web pages, so duty holders routinely buy one expecting the answer only the other can give. The honest comparison is this: they measure different things, for different purposes, and the confusion between them costs money in one direction and creates false assurance in the other.
What a dip slide actually measures
A dip slide is a plastic paddle coated on both sides with a nutrient jelly. You dip it in the water (or run the water over it), put it back in its tube, incubate it, and read the colonies that grow against a comparison chart. What grows is whatever general aerobic bacteria happened to be in the water, so the result is a total viable count: an estimate of the overall microbial population, with no identification of species.
Legionella will not show up. It grows slowly and needs specific nutrients that a general-purpose dip slide does not provide, which is why laboratories culture it on selective media for around ten days instead. A dip slide can come back clean from water that contains legionella, and dirty from water that does not. Neither result tells you anything about the organism you are actually worried about.
What a laboratory culture test measures
A laboratory test sends a one-litre sample to a UKAS-accredited laboratory, where it is filtered, treated and grown on selective media under BS EN ISO 11731. After around ten days the laboratory reports which legionella species and serogroups were found and a count in colony-forming units per litre, which you read against the action levels in our CFU levels guide. This is the only widely accepted way to answer the question “is legionella in this water, and how much?”. Our testing laboratory directory compares accredited laboratories on method, turnaround and price.
The honest comparison
| Dip slide | Laboratory culture | |
|---|---|---|
| What it detects | Total viable count of general aerobic bacteria | Legionella specifically: species, serogroup and count in cfu/L |
| Answers the legionella question? | No | Yes |
| Time to result | Around one to two days (incubation) | Around ten days |
| Cost per test | Pounds; done in-house | Published prices in our directory start from around £45 per sample |
| Typical use | Frequent trend check on treated systems, chiefly cooling towers | Verification, investigation, and the routine sampling set by HSG274 and HTM 04-01 |
| Who reads it | The operator, against a chart | The laboratory, with an accredited report for the log book |
When dip slides are the right tool
Dip slides earn their place on systems that run a continuous water treatment programme, above all cooling towers. The operator needs a cheap check, far more often than a laboratory could be paid to visit, that the biocide regime is holding the general bacterial population down. HSG274 Part 1 builds dip slides into the routine monitoring for exactly this: a weekly trend that flags a slipping treatment programme within days, sitting alongside the quarterly laboratory sampling that measures legionella itself. Our guide on cooling tower legionella control sets out the full regime, and HSG274 explains the structure of the guidance.
The skill is reading a dip slide as a trend, not a verdict. A single slightly raised count means little; a count that climbs week on week tells you the treatment is failing and it is time to act before the laboratory sample confirms it.
Free legionella risk assessment template
A structured Word document following the five-step approach in ACOP L8. Covers risk identification, written scheme, monitoring, and records. If it isn't written down, you can't evidence it.
Follows ACoP L8 and HSG274 Part 2. Free. No spam.
When you need the laboratory instead
If the question contains the word legionella, the dip slide is out of scope. Use laboratory culture whenever you are verifying a control regime where temperature control is not practical, investigating an out-of-range temperature or a case, confirming remediation after a positive result, or meeting a set sampling frequency under HSG274 or HTM 04-01. For most ordinary hot and cold water systems, though, the first line of defence is neither test: it is temperature control, checked with the legionella temperature checker and recorded in the log book. The fuller picture of when sampling is and is not needed is in our guide to legionella testing.
A dip slide tells you how much bacteria is in the water, not which bacteria. It is a trend gauge for a treatment regime, never a legionella test, and a clean slide is no evidence that legionella is absent.
Free legionella risk assessment template
A structured Word document following the five-step approach in ACOP L8. Covers risk identification, written scheme, monitoring, and records. If it isn't written down, you can't evidence it.
Follows ACoP L8 and HSG274 Part 2. Free. No spam.
Frequently asked questions
Do dip slides test for legionella?
No. A dip slide measures the total number of general aerobic bacteria in the water, reported as a total viable count. Legionella is slow-growing and fussy about nutrients, so it does not grow on a dip slide in any usable way, and a dip slide cannot tell you whether legionella is present or absent. Only a laboratory method such as culture to BS EN ISO 11731, or a qPCR DNA test, detects legionella specifically.
What does a dip slide actually measure?
A dip slide is a paddle coated with a general-purpose nutrient jelly. You dip it in the water, incubate it, and compare the colonies that grow against a reference chart to estimate the total viable count of aerobic bacteria. It is a general cleanliness indicator: a rising count tells you microbial activity in the system is increasing, which usually means the treatment or control regime is slipping. It says nothing about which bacteria are present.
Are dip slides accurate?
They are reasonably accurate at the thing they measure, which is the general bacterial population, and that is a genuinely useful trend indicator on treated systems such as cooling towers. The accuracy problem only appears when a dip slide is treated as a legionella test, which it is not. A clean dip slide can sit next to a colonised system, and a dirty one can come from harmless bacteria, so neither result answers the legionella question either way.
Why do cooling towers use dip slides?
Cooling towers run a continuous biocide treatment programme, and the operator needs a cheap, frequent check that the treatment is keeping the general bacterial population down. HSG274 Part 1 sets dip slides into the routine monitoring for exactly this purpose: a weekly trend the operator can act on quickly, alongside quarterly laboratory sampling that measures legionella itself. The dip slide is the smoke alarm for the treatment regime; the laboratory culture is the test for the bacteria that matters.
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