Rabbit Pyrogen Test (Sham Test): Principle, Procedure, Interpretation

Before a batch of injectable saline reaches a hospital ward, someone has to be sure it will not give patients a fever. For most of the twentieth century, the way to be sure was to inject a sample into rabbits and watch their temperature. A rabbit's fever response is close enough to a human's that if the product was pyrogenic, the rabbits would spike a fever within hours. That test is still on the syllabus, still in the pharmacopeias, and still known by a name that confuses students the first time they meet it: the Sham test.

Introduction

The rabbit pyrogen test (RPT) is the original method for detecting pyrogens in injectable products. In pharmacy and industrial pharmacy courses it is widely called the Sham test. Both names refer to the same in vivo (living animal) test. It is a real, formal pharmacopeial test, not a fake or a control run, and the name is a source of needless confusion, which is worth clearing up at the start.

The test works on a simple idea. A pyrogen is any substance that causes fever when it enters the blood. If you inject a test sample into a rabbit and the rabbit's temperature rises, the sample contains pyrogens. If the temperature stays flat, it does not. Because the rabbit is a whole living animal with an intact fever response, the test detects all pyrogens, both bacterial endotoxin and non-endotoxin pyrogens. This is its main advantage over the LAL test, which detects only endotoxin. It is also the reason the rabbit test survived so long: no in vitro method could match its breadth until the monocyte activation test arrived.

The rabbit pyrogen test is defined in USP chapter 151 (Pyrogen Test).

Why the rabbit test is being replaced

The rabbit test has real drawbacks. It uses live animals, which raises ethical concerns. It is slow, taking hours per run and weeks of animal conditioning beforehand. And it varies from rabbit to rabbit, because individual animals differ in how strongly they respond. For all these reasons it is being phased out in favor of in vitro methods: the monocyte activation test (MAT), which also detects all pyrogens, and the LAL and rFC assays, which detect endotoxin. Several pharmacopeias have moved toward the LAL test as the preferred method for endotoxin. The Indian Pharmacopoeia, for example, moved to the LAL test as the preferred endotoxin method in its 2018 edition. The rabbit test has not been deleted, but its role is shrinking.

Principle of the rabbit pyrogen test

The basis of the test is measuring the rise in body temperature of rabbits after a sterile solution of the test sample is injected intravenously. If the sample contains pyrogens, the rabbits' temperature rises. The size of the rise, judged against defined limits, decides whether the product passes or fails.

Procedure

The procedure has two parts: a preliminary test (which selects and conditions suitable rabbits) and the main test (which tests the actual product).

Preliminary test

The preliminary test makes sure the rabbits are healthy, stable, and not already febrile, so that any temperature rise in the main test can be trusted to come from the sample.

1. Select healthy rabbits that have not been used in a pyrogen test for the previous few days, and condition them to the test environment beforehand so they are calm on test day.
2. Withhold food during the test period, and give water as specified. Record each rabbit's baseline temperature. A normal rabbit temperature is around 38.5°C.
3. Inject pyrogen-free saline intravenously as a sham (product-free) run.
4. Record the temperature at 30-minute intervals for about 3 hours.
5. Reject any rabbit whose temperature is unstable or rises significantly during this sham run, and replace it. Only rabbits with stable baseline temperatures proceed to the main test.

Main test

1. Warm the prepared sample to about body temperature (37 to 38°C) before injection, so the injection itself does not disturb the rabbit's temperature.
2. Withhold food from the selected rabbits for the defined period before the test.
3. Record each rabbit's control (baseline) temperature as the mean of readings taken shortly before injection. Do not use a rabbit whose two baseline readings differ by more than 0.2°C, because an unstable baseline makes the result unreliable.
4. Inject the sample slowly into the marginal ear vein of each rabbit. The injected volume is defined per kilogram of body weight and must not exceed the pharmacopeial maximum (not more than 10 ml/kg).
5. Record each rabbit's temperature at 30-minute intervals for 3 hours after injection.
6. For each rabbit, the maximum temperature rise is the highest reading after injection minus its control temperature. Treat any temperature fall as a zero rise.

Interpretation of results

Interpretation depends on the compendium. The most widely cited standard is USP 151, and many pharmacy textbooks also teach the older staged three-rabbit criteria. Both are given here so you can match whichever your syllabus uses.

USP 151 criteria (current compendial standard)

The test begins with three rabbits.

• If no rabbit shows an individual temperature rise of 0.5°C or more, the product passes (absence of pyrogens).
• If any rabbit shows a rise of 0.5°C or more, the test is continued with five more rabbits, making eight in total.
• The product passes if not more than three of the eight rabbits show a rise of 0.5°C or more, and the sum of the eight individual maximum rises does not exceed 3.3°C.
• If more than three of the eight rabbits show a rise of 0.5°C or more, or the sum of the eight rises exceeds 3.3°C, the product fails (pyrogens present).

Staged three-rabbit criteria (older textbook version)

Some textbooks and older pharmacopeial editions use a three-rabbit table with a 0.6°C individual threshold and a summed threshold, repeating in additional groups of rabbits when the result is borderline. In this version:

1. Pass: no rabbit shows an individual rise of 0.6°C or more, and the summed rise across the three rabbits stays under the defined limit.
2. Fail: two or three rabbits show a rise of 0.6°C or more, or the summed rise across the three rabbits exceeds the limit.

Borderline results are resolved by repeating the test in further groups of rabbits. If your course follows the older three-rabbit table, use its exact figures; if it follows USP 151, use the 0.5°C and 3.3°C figures above.

Application

The rabbit pyrogen test is a compendial method for detecting pyrogens in parenteral products, and its defining strength is that it detects both endotoxin and non-endotoxin pyrogens. It has historically been used by pharmaceutical manufacturers to check injectable products before release. Its use is now declining as in vitro methods (MAT for all pyrogens, LAL and rFC for endotoxin) take over, but it remains a reference method and a standard teaching example of an in vivo safety test.

How to Remember

The Sham test is the rabbit test. It is a real pharmacopeial test, not a fake or a control. When a pharmacy question says "Sham test," read it as "rabbit pyrogen test."

Rabbit catches everything. Because the rabbit is a whole animal with a real fever response, it detects all pyrogens, not just endotoxin. That is the one thing the rabbit test does that LAL cannot.

Three, then eight (USP 151) Start with three rabbits. If one crosses 0.5°C, add five more to make eight. Pass needs no more than three of eight over 0.5°C and a total under 3.3°C.

Warm the sample, steady the rabbit. The sample is warmed to body temperature and the rabbit needs a stable baseline (readings within 0.2°C), so that any fever seen must come from the product, not from the handling.

Key exam facts

Where Students Get Confused

"The Sham test is a fake test or a negative control." No. In pharmacy usage, the Sham test is simply another name for the rabbit pyrogen test, the in vivo method. It is a real, formal pharmacopeial test.

"The rabbit test only detects endotoxin." No. That is the LAL test. The rabbit test detects all pyrogens, including non-endotoxin pyrogens, because it measures a real fever response in a living animal.

"The threshold is 0.6°C." It depends on the standard. Current USP 151 uses a 0.5°C individual rise and a summed limit of 3.3°C across eight rabbits. The 0.6°C figure comes from older three-rabbit textbook criteria. Use whichever your syllabus specifies, and know that they differ.

"You always use three rabbits." Not exactly. USP 151 starts with three, but if any rabbit crosses the threshold, the test continues with five more, for eight in total. The final decision can rest on all eight.

"The rabbit test has been banned." No. It is being phased out for ethical and practical reasons and is increasingly replaced by MAT and LAL, but it remains in the pharmacopeias as a reference method.

References

1. United States Pharmacopeia. General Chapter 151 Pyrogen Test.
2. European Pharmacopoeia. Pyrogens (2.6.8).
3. Indian Pharmacopoeia Commission. (2018). Indian Pharmacopoeia.
4. Sandle, T. (2016). Pharmaceutical Microbiology: Essentials for Quality Assurance and Quality Control. Woodhead Publishing.