---
title: "Total viable count (TVC) explained: what microbiological monitoring tells you that a Legionella test doesn't"
source_url: https://legionella.io/articles/total-viable-count-water-testing/
canonical_url: https://legionella.io/articles/total-viable-count-water-testing/
pillar: "Monitoring, Flushing & Sampling"
summary: "TVC measures the total bacteria in your water, not Legionella. Learn what a rising aerobic colony count signals, why 22C and 37C matter, and when to act."
primary_keyword: "total viable count water testing"
date_published: 2026-05-23
date_reviewed: 2026-06-26
author: "Legionella.io editorial team (REMOTE TECH LTD)"
reviewed_against: "HSE L8 and HSG274 guidance"
region: "United Kingdom"
license: "(c) REMOTE TECH LTD. Quote freely with attribution and a link to source_url."
---

# Total viable count (TVC) explained: what microbiological monitoring tells you that a Legionella test doesn't

A total viable count (TVC) is the number of living, culturable aerobic bacteria in a water sample, reported as colony-forming units per millilitre. It measures the general microbiological health of the water. It does not measure Legionella. A Legionella test counts one specific organism; a TVC counts the whole crowd that organism lives among.

That distinction is the source of a lot of confusion on water-analysis and cooling-tower certificates. The two figures sit next to each other, both labelled "CFU", and it is tempting to read a low TVC as proof there is no Legionella. They answer different questions, and treating them as interchangeable lets a problem build unseen.

## What each number is actually counting

A TVC — also called an aerobic colony count (ACC) — is deliberately broad. The lab grows whatever culturable aerobic bacteria are in the sample on a general nutrient medium and counts the colonies that appear. The result is a population estimate for the water as a whole. Low and stable usually means the system is clean and under control. High or climbing means something is feeding microbial growth: nutrient ingress, warmth, stagnation, or a biofilm establishing itself on pipe and tower surfaces.

A Legionella culture is the opposite of broad. It uses selective media and sample preparation designed to recover Legionella species and suppress almost everything else, then enumerates only the colonies confirmed as Legionella. Because the organism is comparatively scarce even in a system that has it, the result is reported per litre, not per millilitre — the lab concentrates a large sample volume to stand any chance of finding it [2].

So a TVC tells you about the water community. A Legionella count tells you about one member of it. A system can return a respectable TVC and still harbour Legionella sheltering inside biofilm or amoebae, and a system with a high TVC may carry no detectable Legionella at all. The two are not proxies for each other — see [Legionella and other pathogens in building water systems](https://legionella.io/articles/legionella-and-other-pathogens-in-building-water-systems/) for why a single count never describes the whole microbial picture.

## Why 22°C and 37°C appear on the report

TVC is usually run at two incubation temperatures, and the certificate normally lists both. The 22°C plate favours the environmental bacteria you would expect in general water and gives a feel for overall cleanliness. The 37°C plate favours organisms that grow at body temperature, a group of more direct interest to health. The pair together is the standard method descriptor. A 37°C count that climbs faster than the 22°C count is worth a second look, because it points to a change in the kind of organisms growing, not just the quantity.

## TVC versus a Legionella count, side by side

| What you're asking | TVC / aerobic colony count | Legionella culture |
|---|---|---|
| What it measures | All culturable aerobic bacteria in the water | Legionella species specifically |
| Reported in | CFU per millilitre | CFU per litre |
| Method descriptor | General-medium plate or dip slide, incubated at 22°C and 37°C | Selective culture to BS 7592 / ISO 11731, around ten days |
| What a result tells you | Overall microbiological control and cleanliness | Whether the pathogen is present, and at what level |
| What it cannot tell you | Whether Legionella is present | General biofilm or nutrient load |
| Best treated as | A frequent trend / baseline indicator | A periodic confirmatory check |

## A single number means little; the trend means a lot

TVC earns its keep as a trend indicator, not a one-off verdict. Establish a baseline for the system, then watch the direction of travel. A steadily rising TVC in a cooling tower is one of the earliest signs that microbiological control is slipping — that the biocide regime, bleed rate or general cleanliness is no longer holding the line. The same conditions that let the general population climb also build the biofilm that Legionella colonises, so a rising TVC trend can flag deteriorating control well before a Legionella sample turns positive. And a Legionella sample only ever reflects one point at one moment; the TVC trend reflects the system over time.

This is why cooling towers are typically monitored with weekly dip slides — a field version of the TVC, read on site over a couple of days — with full Legionella sampling at less frequent, defined intervals [1]. The dip slide is the smoke alarm. The Legionella culture is the confirmatory check.

What a TVC does not come with is a universal pass/fail line. Resist the urge to treat any figure you have seen quoted as a threshold. Your risk assessment and written scheme of control set the action levels for your system, informed by the example trigger levels HSG274 gives for cooling-tower aerobic colony counts [1]. The number that matters is the one your scheme says triggers investigation — and any sharp move away from your own established baseline.

## How to read your certificate without over- or under-reacting

For most hot and cold water systems kept under good temperature control, routine Legionella sampling is not generally expected, and TVC is not a standard requirement on those systems either [3]. TVC monitoring earns its place mainly on cooling towers, evaporative condensers and other higher-risk or harder-to-control systems, where it serves as the practical microbiological indicator.

When both numbers are on the page, read them in order. Treat the TVC trend as the early-warning gauge and the Legionella result as the periodic confirmation. A low Legionella result sitting under a rising TVC is not an all-clear — it is a prompt to investigate the trend before the next sample is due. Equally, a clean TVC does not license you to ignore Legionella, because the organism can persist out of reach of a general count. For how to act on the Legionella figure itself, see [Interpreting Legionella test results and counts](https://legionella.io/articles/interpreting-legionella-test-results-and-counts/) and the units explainer [Understanding lab results: CFU counts and what they mean](https://legionella.io/articles/understanding-lab-results-cfu-counts-and-what-they-mean/). For how the pathogen is specifically isolated and why a culture takes longer than a TVC, see [Detecting Legionella: culture and PCR testing methods](https://legionella.io/articles/detecting-legionella-culture-and-pcr-testing-methods/).

This is general guidance, not a substitute for your site's risk assessment. The action levels, sampling points and frequencies — for TVC and for Legionella alike — are decisions for a competent person who knows your system, working to ACoP L8, HSG274 and BS 7592. Nothing here is legal, medical or system-design advice. A certificate that worries you is a conversation to have with your water-treatment provider, not a number to interpret alone.

## Common questions

### Does a low TVC mean my water is free of Legionella?

No. A TVC is a broad count of general aerobic bacteria, and Legionella can be present at levels a general count will not reveal — particularly when it is sheltered inside biofilm or amoebae. Only a Legionella-specific culture answers that question.

### Is TVC the same thing as an aerobic colony count?

Effectively, yes. "Total viable count" and "aerobic colony count (ACC)" describe the same broad measure of culturable aerobic bacteria, and labs and control schemes use the terms interchangeably. Both are usually reported at 22°C and 37°C.

### Why is TVC measured per millilitre but Legionella per litre?

Because general bacteria are abundant and Legionella is comparatively rare. A single millilitre holds more than enough to count for a TVC, whereas finding Legionella needs a much larger, concentrated volume, so the result is expressed per litre.

### My TVC is rising but Legionella came back negative, so what should I do?

Treat the trend as a control problem, not a clean bill of health. Review the biocide regime, bleed and cleanliness with your provider, and compare the count against your scheme's action level. Acting on the upward trend now is cheaper than waiting for the next Legionella sample to confirm a problem.

## Your next step

Pull your last three or four certificates for the same system and plot the 22°C and 37°C TVC figures in order of date, with your scheme's action level marked as a line. If the trend is flat and below that line, file it. If it is drifting upward — even with a negative Legionella result — raise it with your water-treatment contractor this week and ask what changed in the biocide or bleed regime, rather than waiting for the next scheduled Legionella sample to make the decision for you.

## Sources

[1] HSE, "Legionnaires' disease: Technical guidance (HSG274)". https://www.hse.gov.uk/pubns/books/hsg274.htm
[2] BSI, "BS 7592:2022 - Sampling for Legionella bacteria in water systems. Code of practice". https://knowledge.bsigroup.com/products/bs-7592-sampling-for-i-legionella-i-bacteria-in-water-systems-code-of-practice-1
[3] HSE, "Testing and monitoring your water system for legionella". https://www.hse.gov.uk/legionnaires/testing-monitoring-water-system.htm