Blood product simulants: water, glycerol or a validated block

Why it matters

Air tells you something is happening. The product probe tells you whether it matters

Open a blood fridge door and the air warms within seconds. A red cell unit takes many minutes to follow, because it holds far more heat than the air around it. A probe in air therefore sees every door opening and every defrost, while a probe in a simulated product shows roughly what the blood itself is doing.

That is why good blood storage monitoring uses both. The air probe raises an alert: an early warning, worth a short delay so routine door openings do not train staff to ignore it. The product probe raises an action alarm: a storage limit has been breached at product level, so it should fire at once. The simulant is what makes that second alarm trustworthy, and it is only as good as its likeness to a real unit.

What the standards say

BS 4376 uses water, by volume, and is silent on glycerol

Start with what applies to you. UK regulation does not name a medium. The Blood Safety and Quality Regulations require hospital blood banks to validate their storage processes, and the Red Book sets no alarm set points; it asks for validated storage, a permanent continuous record and a log of alarm events. For medicines rather than blood, NHS SPS suggests propylene glycol in a volume matching the smallest item stored. For transport validation, the ISBT guideline prefers expired units and says water-filled blood packs can be used to simulate red cells.

BS 4376 is different: it is written for the people who make and test blood storage equipment, not for the blood bank. Part 1 (BS 4376-1:1991) covers closed reach-in refrigerators for whole blood and red cells at 4 ± 2 °C, and Part 2 frozen plasma cabinets. In Part 1, the fridge's own dial thermometer and its high and low alarms work on air temperature (clauses 4.10.1 and 4.11), while the sensor for the continuous temperature record sits fully immersed in a container, and the sensor and container together must hold as much heat as 200 mL of water (clause 4.10.2 c; Part 2 uses 100 mL for plasma cabinets). The test method fills that container with about 200 mL of water (A.1.2). Nowhere does Part 1 mention glycerol or glycol.

Part 1 also shows why the two readings differ. During an automatic defrost the air may go above its usual ceiling, provided the water in the recorder's container stays at or below 7 °C (clause 5.1). And it expects the record to be accurate to within half a degree between 0 °C and 10 °C, checked periodically against a calibrated reference (clause 4.10.2 d, A.1.1 and the note to clause 6.4).

Where the container goes is decided by a mapping test. With the cabinet empty but its shelves and drawers fitted, the door shut, and the air controlled between about 2 °C and 6 °C, sensors are placed at top, middle and bottom shelf positions, either side of the evaporator, at the container's position and anywhere else likely to run warm, and recorded for at least 15 minutes. The container then goes in one of the warmest positions that is practical for the design, with a numbered diagram of where each sensor sat (clauses 4.10.2 c and A.3).

Part 2 also expects that a cabinet without its own recorder and alarms will be connected to a central system that provides the same functions. In our view the same principle applies to any external monitoring system: if the product probe stands in for the blood, it should match the thermal mass the standard uses for the record, and sit at the warm spot your mapping finds. Part 1's air alarm is the standard's own design; our recommendation of an action alarm on the product probe goes further than the standard, for the reasons above.

The glycerol question

Glycerol is a habit, not a finding

Glycerol bottles are everywhere in blood banks, and many fridges arrive with one fitted. The wording often traces to North American procedures, which ask for a fluid with heat transfer 'similar to blood (e.g., 10% glycerol)', as in this Ontario template procedure. We looked for the evidence behind that example and found none: no British Standard, MHRA or UKAS document, and no peer-reviewed study showing glycerol represents blood better than a defined volume of water. What we did find was vendor material and forum discussion.

The physics points the other way. Blood is mostly water: plasma is about 92% water (OpenStax) and whole blood around 80% once the water inside red cells is counted. Its heat capacity is close to water's too: about 3.59 J/g·K for whole blood (Blake and colleagues, 2000) against 4.18 J/g·K for water. Most glycerol bottles hold a dilute mixture, typically around 10%, which is about 90% water. Thermally it behaves almost exactly like water, so it is no better a simulant than water, just harder to make up consistently. Strong glycerol mixtures store less heat than blood and slow the probe down, which makes the reading later, not more realistic.

If you want short air swings damped, that comes from mass and container, which you can define and test. A 2018 study of cold chain buffers concluded that buffer size and material both change the reading, and that there was no standard procedure for choosing one.

What a hospital found

A transfusion laboratory put a blood bag and a water bottle side by side

A hospital transfusion laboratory compared a red cell unit with a 200 mL water bottle using Sematics probes; we'll publish the data with their permission.

One lesson applies to any comparison like this. Undisturbed in the fridge, the bottle read a fraction of a degree cooler than the bag, and so did a glycerol container in a separate run. That steady difference is within the probes' own calibration tolerance and uncertainty (BS 4376-1 itself only asks the fridge's record to be within half a degree, clause 4.10.2 d), and the probes were not cross-checked against each other before the test, so it can't be separated from calibration offset. It isn't evidence of a real difference between the media without a side-by-side check of the probes, or each probe's certificate corrections applied.

The comparison that matters is the response: how soon each one warms past a limit when taken out, how high it peaks, and how soon it recovers. Response is far less sensitive to a fixed probe offset, although even timing comparisons assume the probes respond alike. On response, the tall 200 mL bottle was slower than the bag: it warmed past 6 °C later, peaked lower and came back below 6 °C sooner, so it was the less cautious of the two. We'll also say, with the data, whether each probe sat on the bag's surface or in its core, because storage is defined by core temperature (Red Book 7.3.2.3) and a surface probe responds faster than the core.

Solid blocks

A solid simulant that behaves like 200 mL of water

Bottles leak, evaporate and get refilled by eye, and a fill line drawn on in 2019 rarely survives. A solid block avoids that, but only if it has been shown to behave like the reference it replaces. Our own bench test in 2010 put a simulant block next to 200 mL of water and a 100 mL water bag through a door-open warm-up: the block tracked the 200 mL water closely, while the smaller bag warmed faster than both.

Whatever you use, ask for the comparison data, not a description. A simulant that has been validated against a 200 mL water reference, in a fridge like yours, is a stronger position than any medium chosen by habit.

Shape and volume

200 mL in a tall bottle is not 200 mL in a flat bag

Heat moves in through the surface, so the same volume responds at different speeds in different shapes. A red cell unit is flat, with a lot of surface for its volume. A tall, narrow bottle has much less, and the probe sits further from the surface. The hospital data shows it: in both disturbance runs the flat bag warmed faster than the 200 mL bottle.

So fix the volume and the container together, write both down, and use the same container for every fridge of the same type. Changing from a squat pot to a slim bottle, or topping up by eye, quietly changes your alarm's timing. Fill volume matters as well: BS 4376 uses 200 mL equivalent for refrigerators and 100 mL for plasma cabinets, and a smaller simulant than your smallest unit will respond faster than the blood.

Test it yourself

A comparison test you can run in an afternoon and a night

You do not need a laboratory to settle this for your own equipment. Use an out-of-service fridge, calibrated probes and a discard unit.

• Cross-check the probes. Calibrate or cross-check all the probes together first, in the same bath at the same time. Record each probe's correction, and apply it to every reading before you compare the media.
• Set up. Place a discard red cell unit, 200 mL of water in your standard container, your current glycerol or glycol bottle, and any block you are considering, side by side at shelf height. Put a calibrated probe in or on each simulant, the same way you would in service. For the discard unit, measure the core: insert the probe through a port, which is acceptable because the unit is discarded. If you can only measure the surface, record that, because the surface responds faster than the core.
• Stabilise. Run the fridge at about 4 °C for at least 12 hours with the door shut. Note any steady offset between probes after corrections; treat an offset within the probes' calibration uncertainty as no difference, and swap two probes and repeat if you need to separate probe error from the media.
• Disturb. Hold the door open for a fixed time, or switch the fridge off for a fixed period, and log at 1-minute intervals if your system allows.
• Measure. Base the comparison on response. For each simulant, note the time to pass 6 °C, the peak, and the time to get back below 6 °C, all measured from the same moment and against the same threshold, compared with the bag.
• Decide and record. Choose the simulant whose response is closest to the bag, or slightly faster. Write the result, the container and the fill volume into your validation file.

Our recommendation

A water-equivalent simulant that responds no slower than your units, and the justification on file

Your trust or blood bank owns the justification for its choice, and UKAS assessors and MHRA inspectors may ask for it. This is the position we would defend.

How we can help

We supply simulant blocks that tracked 200 mL of water closely in our own bench tests, and we can help you run the comparison test on your own fridges with Sematics logging at 1-minute intervals. We calibrate the temperature probes, in our UKAS accredited calibration laboratory No. 10723 where you need UKAS, or cross-check them together before the test, so each probe's correction is known before you compare the media.