Many antibodies used in diagnostic immunoassays are derived from rabbits. Keeping rabbits as pets is known to be a risk factor for developing heterophilic (or interfering) antibodies.1 Studies have shown that 30-40% of the population have heterophilic antibodies.2 However, only about 0.05-0.5% of immunoassays seem to be affected to the extent that the concentration of interfering antibodies overwhelms the assay system.2 We report a case in which the presence of heterophilic antibodies led to unnecessary investigations.
Case history
A 52 year old woman was referred to our hospital in July 2001 for further investigation of persistently raised fasting gut hormones concentrations. She had had irritable bowel syndrome diagnosed 16 years previously. The high concentrations of gut hormones had first been detected nine years ago, when, after an exacerbation of her condition, she had investigations to screen for other possible causes of diarrhoea. Computed tomography of the abdomen, magnetic resonance imaging of the pancreas, and an octreotide scan at that time all gave normal results. The referring hospital attributed the abnormal blood test results to hyperplasia of pancreatic islet cells.
The patient was monitored with two yearly magnetic resonance imaging and annual measurement of fasting gut hormone concentrations. The results of imaging were always normal apart from a suggestion of hyperplasia of the islet cells, but her gut hormone concentrations remained persistently high. She was receiving no drugs apart from an oestrogen implant. She was managed conservatively as her condition remained stable.
On referral to our hospital, the patient's fasting gut hormone concentrations were still high (see table). Imaging of the abdomen and octreotide scanning gave normal results. We therefore booked the patient for pancreatic angiography with calcium stimulation to ascertain whether she had abnormal functioning islet cells.3
It is unusual for a neuroendocrine tumour to secrete more than one hormone, so we considered whether the patient could have heterophilic (interfering) antibodies. The radioimmunoassays for fasting gut hormones all use rabbit antibodies. On further questioning, it transpired that she and her husband had kept large numbers of pet rabbits (up to 80 at one time) and that she had presumed rabbit induced allergic rhinitis. Her husband was an officer of the British Rabbit Council.
The presence of heterophilic antibodies in the patient's serum was confirmed by the addition of small concentrations of non-immune rabbit serum to the gastrin assay buffer. This blocks the interfering antibodies without otherwise affecting the assay. The patient's results for gastrin were 95 pmol/l (no rabbit serum added), <20 pmol/l (0.5% rabbit serum added), and <20 pmol/l (1% rabbit serum added). The reference range for gastrin is <30 pmol/l. We cancelled the angiography, reassured the patient, and discharged her back to the referring hospital with the diagnosis of presumed irritable bowel syndrome with heterophilic (rabbit) antibodies interfering with the gut hormone assay.
Discussion
It is likely that our patient's exposure to her pet rabbits led to the development of the heterophilic antibodies. The consequence of this in her case has been needless investigations and clinic appointments, although, fortuitously, no major clinical intervention. Earlier communication may have led to the interference being detected sooner.
Heterophilic antibodies are common in the population and cause interference in up to 0.5% of immunoassays.1,2 Interference from heterophilic antibodies should be considered whenever immunoassay results do not correspond with the clinical and diagnostic pictures, and it is important to recognise that multiple immunoassays can be affected. Failure to take account of heterophilic antibodies can result in unnecessary investigations and clinical interventions. It is therefore essential that physicians and the laboratory interact closely. Increased use of assays with heterophilic antibody protection would also help avoid the problem.
Figure.
People who keep rabbits may develop heterophilic antibodies
Table.
Patient's fasting gut hormone concentrations (pmol/l), 1993-2001
| Hormone (reference value)
|
January 1993
|
April 1995
|
April 1996
|
April 1997
|
April 1998
|
June 1999
|
March 2000
|
July 2001
|
|---|---|---|---|---|---|---|---|---|
| Vasoactive intestinal polypeptide (<30) | 279 | 30 | 7 | <4 | 86 | 35 | 38 | 120 |
| Pancreatic polypeptide (<300) | >500 | >500 | 116 | 300 | >500 | >500 | 400 | 72 |
| Gastrin (<30) | >400 | >400 | >400 | >400 | >400 | >400 | >400 | 125 |
| Glucagon (<50) | >500 | 93 | 59 | 44 | 56 | 27 | 34 | 63 |
| Somatostatin (<150) | 71 | 284 | 247 | 92 | 156 | 93 | 138 | 103 |
| Neurotensin (<100) | >1000 | >1000 | >1000 | 752 | >1000 | 718 | 248 | 101 |
Interfering antibodies must be considered when the clinical picture and immunoassay results do not match
Footnotes
Funding: None.
Competing interests: None declared.
References
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