Time-temperature indicators: how they work and what they can show

Types of temperature indicator

Threshold indicators

labels that change once a set temperature is crossed, upward for heat or downward for cold, and stay changed. WHO describes them as irreversible and typically single-use.

Progressive indicators

labels that register repeated events cumulatively. Nothing changes while the temperature stays below the threshold; each time it is exceeded the change advances, usually as color moving along a paper strip.

Time-temperature integrators

labels whose chemical reaction or diffusion runs faster as the temperature rises, so they measure accumulated heat rather than a single spike. The change cannot be undone by cooling, and the color development can be matched to the known stability of the product.

Freeze indicators

labels or small devices that show a product has been exposed to freezing, which some vaccines cannot tolerate. WHO's prequalification program says they should be used routinely in the internal distribution of freeze-sensitive vaccines, and its specification for chemical freeze indicators requires the label to change irreversibly within 60 minutes at temperatures colder than −0.5 °C.

Electronic temperature indicators

compact battery-powered devices with a sensor, a microprocessor, and memory that show whether an alarm limit was breached, typically on a small display.

An indicator is specified by its limit and, for many, by a time: how long the product can be above the limit before the indicator responds. Choosing one means matching both to the product's labeled range and its stability data. Most come with standard limits; WHO notes that some customization is available for high-volume use.

Vaccine vial monitors and how they are read

The best known time-temperature integrator is the vaccine vial monitor, a heat-sensitive label on the vial itself whose color changes with the heat the vial has accumulated. WHO classes it as a chemical time-temperature integrator, and its prequalification program notes that vaccine vial monitors, or combined vial monitor and threshold indicators, are now routinely fixed to all vaccines supplied by UNICEF. Because it sits on the vial, it carries the product's exposure history past the point where shipment-level monitoring stops.

It is read by comparing the inner square with the circle around it. While the square is lighter than the circle, the vial has not reached the end point. Once the square matches the circle, or is darker, WHO's specification says the vaccine should no longer be used. Before that, a square that has started to darken shows which vials have had some exposure, so they can be used ahead of those that have not.

What an indicator cannot show

WHO's comparison of monitoring device types lists the limits of indicators alongside their benefits:

When, and for how long

an indicator has no time-specific record, so it does not show when an excursion happened or how long it lasted.

Data to analyze

threshold, progressive, and electronic indicators are not an analytical tool: they give nothing to trend a lane or investigate a cause.

Both limits at once

watching an upper and a lower limit together takes two indicators.

A reading free of judgment

the interpretation of a color change can be affected by human factors.

In practice this means a tripped indicator starts a decision rather than ending one. The assessment of an excursion turns on how far and how long the product was out of range, measured against its stability data, and an indicator does not record either.

How that assessment works is in our guide to temperature excursions.

Reading an indicator on receipt

WHO's guidance is that the go or no-go result of an indicator is recorded on the product's arrival report, and that the result decides whether the consignment goes into storage or into quarantine until it has been investigated and a final disposition made. An indicator is only as good as that step: one that is not checked, or not written down, leaves no evidence at all.

Indicators printed with a scannable code close part of that gap. Scanning the code with a phone records the reading, so it can be kept with the rest of the shipment's record instead of on a paper form.

Where indicators fit

Because a chemical indicator is cheap, battery free, and needs nothing to read it, it can go where a recording device on every unit would not: on each carton, each pack, each vial. It is most useful alongside a device that records the full history at shipment or pallet level, so that a tripped indicator can be checked against a record of what happened.

How indicators compare with data loggers and real-time devices is in our guide to cold chain monitoring devices.

Where Controlant fits

After the pallet break, cartons and packs travel on to pharmacies, clinics, or patients. Controlant offers QR indicators and Bluetooth labels that go with them, described on our page on labels and indicators. The QR indicator is single use and battery free, and scanning its code on receipt shows whether it was exposed above its limit; bringing that reading into the Controlant Platform is in development. At shipment level, Saga devices record the full history and report it in real time.