To investigate whether serum DAO levels can be used as a marker of elevated mast cell count per mm2 in women with bladder pain syndrome.

Diamine oxidase (DAO) is the major enzyme which metabolises both exogenous and endogenous histamine by removal of the amine group via oxidative deamination [1]. Histamine is the major chemical leading to bladder pain and inflammation. DAO levels can be readily measured in serum and stool samples and there has been increasing interest in correlating DAO levels with the symptoms of histamine intolerance (HIT). HIT is defined by the World Allergy Association as a hypersensitivity to foods containing high levels of histamine without IgE reaction [2]. Mast cells contain large amounts of histamine.  Histamine release and elevated mast cell counts (MCC) in bladder tissue have been implicated in women reporting chronic bladder pain and women with high MCC are often initiated on a low histamine diet. DAO deficiency has previously been found with HIT but has not been associated with mast cell activation [3].
This is the first study exploring whether serum DAO levels can be used as a marker of bladder tissue MCC, reducing the need for invasive cystoscopy and biopsy procedures in patients with bladder pain syndrome.

Women were recruited with bladder pain syndrome who were undergoing cystoscopy and bladder biopsy procedures for serum DAO level to be taken pre-operatively. Mast cell count was given routinely in histology reports using a CD117 stain and a count using mm2. Mast cell count per mm2 and serum DAO levels were analysed for correlations using SPSS software. The upper limit of normal is defined as 28 per mm2.

194 women were recruited from a tertiary urogynaecology unit for this study with a mean age of 46 and parity of 1.2. On statistical analysis, Spearman’s coefficient demonstrated a statistically significant but weakly positive correlation between serum DAO level and MCC rs= 0.173, p=0.009. 
Linear regression modelling shows that DAO level can predict MCC per mm2 with statistical significance F (1, 223) = 6.729 (p=0.01). However, as R2= 0.025, DAO level only accounts for 2.5% of the explained variability in MCC.
When clustering DAO levels into low <10, middle 10-20, and high values >20- showed that with serum DAO >20 we were more likely to see high mast cell counts of >28 per mm2 (see figure 1).
A receiver operating curve (ROC) was used to assess the performance of a serum DAO level of >20 as a test of a high MCC in bladder tissue defined as MCC >28 per mm2. We found that serum DAO cannot be used as a reliable test of MCC with the area under the curve AUC= 0.62 and serum DAO of 20 having a sensitivity of 41.1% and a specificity of 24% (see figure 2).

We have shown that there is a linear correlation between serum DAO levels and bladder tissue MCC in women with bladder pain syndrome, and statistically significant linear regression modelling is possible. However, this relationship is weak and does not translate into serum DAO being a strong marker of elevated MCC in bladder tissue as demonstrated with linear regression modelling and a receiver operating curve.

Our study is the first to study whether serum DAO levels correlate to mast cell counts in women with bladder pain syndrome. Our results show that serum DAO cannot replace cystoscopy and bladder biopsy as a key investigation in women with bladder pain. Therefore cold cup biopsy and staining the tissue for mast cells per mm2 remains an important technique for the investigation of women with bladder pain syndrome.

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