A novel cytologic sampling technique to diagnose subclinical endometritis and comparison of staining methods for endometrial cytology samples in dairy cows

The present article describes a study of the diagnosis of subclinical endometritis in dairy cows having two principal aims: first, to validate a novel technique for taking endometrial cytology samples to diagnose subclinical endometritis in dairy cows. Second, to compare the percentage of polymorphonuclear cells (PMNs) in cytology samples stained with Diff-Quik versus a specific staining method for PMNs, naphthol-AS-D-chloroacetate-esterase (CIAE). In the first experiment, Holstein-Friesian cows (n = 204) were used to take two cytology samples at the same time using the conventional cytobrush (CB) and the new cytotape (CT). Both devices were assembled within the same catheter allowing sampling at the same time, and approximately at the same location. Cytotape consisted of a 1.5-cm piece of paper tape rolled on the top of an insemination catheter covered with a double guard sheet. Parameters used to evaluate both methods were: PMNs percentage, total cellularity, quality of the smears, and red blood cell contamination. The concordance correlation coefficient analysis was used to assess agreement between continuous and Pearson chi-square tests for categorical variables. Agreement between the percentage of PMNs in both methods was good ρ = 0.84 (0.79, 0.87) with a minor standard error of 2%. Both methods yielded similar total cellularity (P = 0.62). Cytotape yielded better quality smears with more intact cells (P < 0.01) while samples that were taken by CB were more likely to be bloody (P < 0.01). Hence, CT and CB methods yielded smears with a similar PMNs percentage and a total number of cells, but CT provided smears with higher quality and significantly less blood contamination. For the second experiment, 114 duplicate cytology slides were stained using both Diff-Quik and CIAE. Agreement between PMNs percentage in both staining techniques was good ρc = 0.84 (0.78, 0.89) with a standard error of only 2%. Hence, Diff-Quik was confirmed as an easy, fast, and high-quality staining technique, which can be routinely used to stain endometrial cytology samples satisfactorily.

Endometrial echotexture variables in postpartum cows with subclinical endometritis

The aim of this study was to evaluate endometrial echotexture changes on ultrasonographic digital images during subclinical endometritis using a computer-assisted image analysis program. Endometrial samples were collected from 140 Brown Swiss cows (days in milk = 35 ± 3) using a cytobrush method and classified as having a non-inflamed uterus (n = 66) and uterus with acute (n = 42), subacute (n = 21), and chronic (n = 11) inflammations. The mean cellular infiltration density was 0%, 31 ± 5%, 37 ± 6%, and 16 ± 8% for cows with non-inflamed uterus and cows with acute, subacute, and chronic uterine inflammations (P < 0.0001). As the cell infiltration density increased, both cervical diameter and mean gray level did not change. There were a liner decrease in homogeneity and a linear increase in contrast in response to increased cellular infiltration density. The sensitivity and specificity were 79.73% and 46.97% for the homogeneity value and 59.46% and 69.70% for the contrast value, respectively. In conclusion, monitoring endometrial echotexture alterations, especially homogeneity and contrast, changed depending on the cellular density and inflammation status and may be potential diagnostic markers for subclinical endometritis in cows.

Characterization of cytokine gene expression in uterine cytobrush samples of non-endometritic versus endometritic postpartum dairy cows

To better understand uterine inflammation in postpartum dairy cows we collected sequential cytobrush samples at 29-35 and at 49-55 d in milk (DIM). Based on the uterine cytology, cows were classified as Non-endometritic (n = 23; <18% neutrophils) or Endometritic (n = 12; ≥18% neutrophils) at 29-35 DIM and Non-endometritic (n = 17; <10% neutrophils) or Endometritic (n = 9; ≥10% neutrophils) at 49-55 DIM. Cows defined as Sham Controls (n = 6) were examined by vaginoscopy at 29-35 DIM and identified as Non-endometritic (<10% neutrophils) at 49-55 DIM. Cytokine gene expression in cytobrush samples was assessed using qRT-PCR. Sham Controls did not differ significantly (P > 0.17) from Non-endometritic cows at 49-55 DIM and these data were combined (n = 23). Uterine cytology-based classification using the aforementioned thresholds effectively separated cows into groups with Endometritic cows having significantly higher expression of pro-inflammatory (interleukin (IL)-1α, IL-1β, IL-6, IL-8, IL-17A CSF-1; P < 0.01) and regulatory (IL-1RA and IL-10; P < 0.03) cytokines, relative to Non-endometritic cows. Furthermore, Non-endometritic cows showed a significant decline (P < 0.03) in the expression of pro-inflammatory (IL-1α, IL-6, IL-8) and regulatory (IL-10) cytokine genes as the postpartum period progressed; whereas Endometritic cows exhibited a sustained elevation in transcript abundance throughout the sample period for both pro-inflammatory and regulatory cytokine genes. Expression of transforming growth factor (TGF) genes was more complex with TGF-β3 expression significantly (P < 0.01) lower at 29-35 DIM and TGF-β1 gene expression significantly (P < 0.03) increased at 49-55 DIM in Endometritic versus Non-endometritic cows. Expression of TGF-β2 gene was 2.7-fold higher (P < 0.01) at 29-35 DIM in cows that remained Endometritic when compared to cows recovering by 49-55 DIM. Some Non-endometritic cows (n = 4) at 29-35 DIM were reclassified as Endometritic at 49-55 DIM. The sampling procedures at 29-35 DIM did not alter either the cellular response (P > 0.43) or cytokine gene expression (P > 0.17) at 49-55 DIM. In conclusion, normal uterine involution is characterized by a progressive decline in pro-inflammatory and regulatory cytokine gene expression, while cows with endometritis show a dysregulated inflammatory process characterized by a sustained elevation in pro-inflammatory and regulatory cytokine gene expression. This analysis also shows that decreased TGF-β2 gene expression at 29-35 DIM may be an indicator of recovery from endometritis.

Uterine inflammation and fertility of beef cows subjected to timed AI at different days postpartum

The aim of this study was to evaluate the uterine health and fertility of postpartum beef cows subjected to timed AI (TAI) protocols at different days from calving. Lactating Nelore cows (Bos indicus; n = 244) were used in this study. The TAI protocols were initiated between 20 and 60 days postpartum (DPP). Cows were subjected to an estradiol-progesterone based TAI protocol. The cows were divided into three groups according to the days postpartum at the time the hormonal treatment was initiated: 1) Early (n = 64), cows between 20 and 30 DPP; 2) Middle (n = 115), cows between 31 and 45 DPP; and 3) Late (n = 65), cows between 46 and 60 DPP. At Day 0 of the protocol, endometrial cytobrush samples were collected. Slides for polymorphonuclear (PMN) cell counting were prepared and cytobrush was used for RNA extraction and analysis of relative abundance of il1, il6, il8, and tnf mRNA. Cows from the Early group had less (P < 0.05) pregnancies per AI (P/AI) than cows from Middle and Late groups; 29.7% (19/64), 45.2% (52/115), and 52.31% (34/65), respectively. Accordingly, the Early group had a greater (P < 0.05) proportion of PMN cells in the uterus than Middle and Late groups; 9.0%, 3.8%, and 2.2%, respectively. Relative abundances of il1 and il8 mRNA were greater (P < 0.05) in the Early group than Middle and Late groups. These results indicate that beef cows subjected to TAI protocols early postpartum (< 30 DPP) are less likely to become pregnant, which is associated with increased inflammation and relative abundance of mRNA for the proinflammatory cytokines il1 and il8.

A comparative analysis of the intrauterine transcriptome in fertile and subfertile mares using cytobrush sampling

Background

Subfertility is a major problem in modern horse breeding. Especially, mares without clinical signs of reproductive diseases, without known uterine pathogens and no evidence of inflammation but not becoming pregnant after several breeding attempts are challenging for veterinarians. To obtain new insights into the cause of these fertility problems and aiming at improving diagnosis of subfertile mares, a comparative analysis of the intrauterine transcriptome in subfertile and fertile mares was performed. Uterine cytobrush samples were collected during estrus from 57 mares without clinical signs of uterine diseases. RNA was extracted from the cytobrush samples and samples from 11 selected subfertile and 11 fertile mares were used for Illumina RNA-sequencing.

Results

The cytobrush sampling was a suitable technique to isolate enough RNA of high quality for transcriptome analysis. Comparing subfertile and fertile mares, 114 differentially expressed genes (FDR = 10%) were identified. Metascape enrichment analysis revealed that genes with lower mRNA levels in subfertile mares were related to ‘extracellular matrix (ECM)’, ‘ECM-receptor interaction’, ‘focal adhesion’, ‘immune response’ and ‘cytosolic calcium ion concentration’, while DEGs with higher levels in subfertile mares were enriched for ‘monocarboxyl acid transmembrane transport activity’ and ‘protein targeting’.

Conclusion

Our study revealed significant differences in the uterine transcriptome between fertile and subfertile mares and provides leads for potential uterine molecular biomarkers of subfertility in the mare.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07701-3.

Diagnostic evaluation of subclinical endometritis in dromedary camels

The objective of the present study was to evaluate the efficacy of various diagnostic methods and to estimate the prevalence of bacterial pathogens associated with subclinical endometritis (SCE) in dromedary camels. During two consecutive breeding seasons, a total of 2122 infertile female dromedaries were assigned to this study and suspected cases of SCE were identified using the established criteria which included failure to conceive after three or more consecutive matings with a fertile male, a clinically healthy genital system, no observable vaginal discharge, and normal sexual behavior. Manual vaginal examination, Metricheck, bacteriological examination using endometrial swabbing, and hemogram assessments were conducted and there were comparisons of results to when there was cytological examination using the Cytobrush technique as the gold standard. The threshold value for positive cases of SCE was set at ≥ 5% polymorphnuclear cells in the cytological samples. Subclinical endometritis was diagnosed in 211 9.94 %) of the total infertility cases. Endometrial swabbing was a more sensitive and specific technique for diagnoses compared with the other methods. Bacillus sp., Staphylococcus sp., and Candida albicans were the most commonly isolated microorganisms. Hemogram testing and rectal and ultrasonographic examinations were not effective for the diagnosis of SCE. It was concluded that, compared with other diagnostic tests, bacteriological examination is more sensitive and specific for the detection of SCE in dromedaries.

Uterine lavage is efficient to recover endometrial cytology sample and does not interfere with fertility rate after artificial insemination in cows

Productivity rates directly depend on the fertility of a herd, which in turn can be influenced by many factors. Semen deposited in the female reproductive tract is foreign to the body and, in response to this invasion, produces an inflammatory reaction, which is characterized by rapid infusion of polymorphonuclear (PMN) cells. Techniques to obtain an endometrial sample are usually invasive and can mask the true inflammatory response. Ultrasound is a noninvasive technique and can contribute to the diagnosis of postartificial insemination (AI) inflammatory response in cattle. The present study was divided into two experiments. The aim of experiment 1 was to compare two methods of endometrial cytology collection, uterine cytobrush (UC) and uterine lavage (UL), and their effects on uterine hemodynamics that provide information about blood flow. The two methods were evaluated by Doppler ultrasound using the spectral and color modes. For that purpose, 19 Nellore cows were synchronized for timed AI and subjected to UC (n = 9) or UL (n = 10). The techniques were performed 4 hours after AI. The results showed that both techniques allow collection of a good quality sample and with enough PMN cells to perform counting. More PMN cells were obtained by UL than UC. There was no difference in uterine blood flow between the UC and UL groups in any of the periods evaluated (34 hours before and 4, 24, and 48 hours after collection of uterine sample). On the basis of results of experiment 1, the effect of UL on fertility was studied in experiment 2. A total of 128 Nellore cows were synchronized for TAI; 35 cows were subjected to endometrial cytology by UL 4 hours after AI, and 93 were not submitted to any procedure (control). Pregnancy diagnosis was performed by transrectal ultrasound 30 days after AI. Pregnancy rates did not differ between UL (54.29%) and control (56.99%) groups. The results of this study showed that UL allows the collection of more representative cells of the surface of the uterus than UC technique and causes no damage to the reproductive tract. Moreover, UL did not affect pregnancy rate when performed 4 hours after AI.

A reliable method using the cytobrush for diagnosis of subclinical endometritis in dairy cattle during the late lactational period

This experiment was performed to assess reliability of the cytobrush-cytology method (CCM) in diagnosis of subclinical endometritis (SCE) using the biopsy-histopathology method (BHM) as a reference in late lactating dairy cows. Reproductive organs were collected from 115 slaughtered multiparous crossbred cows culled due to infertility 398 ± 135 days subsequent to parturition. Samples were collected from the dorsal part of the corpus uteri for analyses. Inflammation status was graded histopathologically based on the cell percentages [(neutrophils, eosinophils, lymphocytes (LYM), macrophages (MAC), and plasma cells)]. Data were subjected to Friedman's test for group comparisons (method and diagnosis), concordance correlation and chi-square tests for consistency of results among methods, and the receiver operating characteristics curve analysis for reliability of the CCM. Percentages of LYM (2.67x) and MAC (3.00x) were greater when evaluated using BHM than with CCM (P < 0.05 for both). The agreement (Cohen's κ value) of results among methods was 0.79 ± 0.06. The sensitivity (Se) and specificity (Sp) of the CCM for defining endometrial inflammation were 79.3% and 100%, respectively. Among inflammatory cells, proportions of LYM and MAC in the CCM had merit for evaluation of uterine inflammation, with an Se of 74.1 and 84.5 and an Sp of 93.0 and 75.4 at the cut-off > 4 and > 0, respectively. The results indicate the CCM may be used in the diagnosis of SCE when the LYM and MAC percentages are considered in chronically infertile cows in the later stages of the lactational period.

Identification of genes associated with susceptibility to persistent breeding-induced endometritis by RNA-sequencing of uterine cytobrush samples

This study aimed to investigate the susceptibility to persistent breeding-induced endometritis (PBIE). Cytobrush samples were collected from 81 broodmares 1-3 days before artificial insemination (AI). Susceptibility to PBIE was evaluated by the presence of ≥ 2 cm of intrauterine fluid 24 h after AI, besides the fertility was determined by a sonographic pregnancy diagnosis 2 weeks after ovulation. RNA expressions were compared between susceptible non-pregnant (SNP) mares (n=9) and resistant pregnant (RP) mares (n=9) as well as between susceptible pregnant (SP) mares (n=9) and susceptible non-pregnant (SNP) mares. 66 differentially expressed genes (DEGs) were identified between SNP and RP mares and 60 DEGs between SP and SNP mares. In SNP compared to RP mares, transcript levels of genes regulating steroid hormone metabolism and neutrophil chemotaxis were lower, while higher for genes participating in uterine inflammation.Transcripts of genes related to extracellular matrix degradation, tissue adhesions, and fibrosis were lower in SP mares than in SNP mares, while higher for genes related to uterine cell proliferation, differentiation, and angiogenesis in SP mares than SNP mares. In conclusion, increased transcript levels of apolipoprotein E (APOE) and roundabout 2 (ROBO2), cluster domain 44 (CD44), integrin beta 3 (ITGB3), and epidermal growth factor (EGF) are possible biomarkers for susceptibility to PBIE. While higher expression of fibroblast growth factor 9 (FGF9), kinase domain receptor (KDR), and C-X-C motif chemokine ligand (CXCL) 16, collagen type V alpha 2 (COL5A2) and fibronectin (FN1) are suggested indicators of fertility in susceptible mares if they receive proper breeding management.

Comparison of the Cervex-Brush alone to Cytobrush plus Cervex-Brush for detection of cervical dysplasia in women with a transformation zone type 3

INTRODUCTION

Twenty percent of colposcopic assessments are inadequate due to a type 3 transformation zone (TZ3). In the absence of colposcopic or histological assessment, subsequent management is guided by the referral screening test. In the UK, routine cervical screening is completed by a Cervex-Brush alone. This study examines the effectiveness of a Cytobrush in addition to a standard Cervex-Brush when used in TZ3 assessment.

METHODOLOGY

An 18-month diagnostic accuracy study in a single National Health Service (NHS) Trust. Women with a TZ3 booked for large loop excision of the transformation zone (LLETZ) with a referral cytology of high-risk HPV and squamous dyskaryosis were recruited. Immediately prior to LLETZ, a Cervex-Brush plus Cytobrush liquid-based cytology sample was taken. Presence of endocervical cells was compared. Predictability of high-grade cervical intra-epithelial neoplasia (CIN2+) was by blind standardised reporting of the LLETZ histology.

RESULTS

One hundred and five women were recruited from a total eligible population of 153 cases (68.8%). Endocervical cell yield was increased with the Cervex-Brush plus Cytobrush when compared to the Cervex-Brush alone (99.1% vs 79.1%, P < .001). Irrespective of cytological grade, age or parity, there was no difference in predictability of CIN2+ between sampling methods.

CONCLUSIONS

When compared to Cervex-Brush sampling alone, the addition of a Cytobrush improves endocervical sampling but does not improve cytological predictability of CIN2+ in women with a TZ3. These data suggest that women who will reliably attend for cytological follow-up can be safely referred to primary care for a Cervex-Brush alone.

Impact of subclinical mastitis on uterine health, reproductive performances and hormonal profile of Zebu × Friesian crossbred dairy cows in and around Jimma town dairy farms, Ethiopia

A longitudinal observational study was carried out from January 2020 to July 2021 to assess the impact of subclinical mastitis (SCM) on reproductive performance and its association with uterine health of crossbred dairy cows. The California Mastitis Test (CMT) and cytobrush technique were used to screen subclinical mastitis and subclinical endometritis, respectively. Milk samples positive for subclinical mastitis were subjected to bacteriological analysis. Data from 84 clinically healthy cows collected and analyzed. The present study revealed a prevalence of subclinical mastitis of 51.2% (43 of 84). The mean days from calving to first service interval were significantly longer in subclinical mastitis positive cows than negative (control) cows (120.51 ± 24.5 and 85.15 ± 28.3, respectively) (P < 0.05). The mean number of services per conception was significantly higher in positive cows (2.51 ± 0.83) than in negative cows (1.59 ± 0.81) (P < 0.05). Lower conception and pregnancy rates at first services were observed in subclinical mastitis cows. Risk factors analysis revealed that prevalence of subclinical mastitis significantly differed with the parity and body condition score (P < 0.05). The current study revealed that subclinical mastitis was significantly and directly associated with subclinical endometritis (P < 0.05). Subclinical mastitis significantly decreased (P = 0.000) progesterone concentrations and increased (P = 0.001) the cortisol concentrations. Staphylococcus aureus were the most predominant bacterial isolates from subclinical mastitic milk, followed by coagulase negative staphylococci (CNS) and streptococci. This study concludes a high prevalence of subclinical mastitis caused by Staphylococcus aureus could inflict harmful effects on reproductive performance of dairy cows, emphasizing the relevance of mastitis control programs in dairy farms.

Cytobrush and cotton swab as sampling tools for molecular diagnosis of female genital schistosomiasis in the uterine cervix

Female genital schistosomiasis (FGS) caused by Schistosoma haematobium is a neglected chronic parasitic disease. Diagnosis relies mainly on a colposcopy, which reveals non-specific lesions. This study aimed to assess the performance of two sampling methods for the molecular diagnosis of FGS in the uterine cervix. We conducted a descriptive cross-sectional study in women of reproductive age in Saint Louis, Senegal, who presented for cervical cancer screening. Cotton swab and cytobrush samples were collected from the cervix and examined by real-time PCR. The PCR results obtained using the cotton swabs were compared with those obtained using cytobrush. Of the 189 women recruited, 56 (30%) were found to be positive for S. haematobium infection via real-time PCR. Women aged 40-54 years were predominantly infected (45%) followed by those aged 25-39 years (36%). Numerically more PCR-positive specimens were identified using cytobrush sampling. Of the 89 women who underwent both cytobrush and cotton swab sampling, 27 were PCR-positive in the cytobrush sampling vs 4 in the swab sampling. The mean Ct-value was 31.0 ± 3.8 for cytobrush-based PCR vs 30.0 ± 4.4 for swab-based PCR. The results confirm that real-time PCR can detect Schistosoma haematobium DNA in the uterine cervix. The next step will be to compare PCR with the other diagnostic methods of FGS.

Classical and Molecular Methods for Evaluation of Chlamydia trachomatis Infection in Women with Tubal Factor Infertility

BACKGROUND

Chlamydia trachomatis is the most reported bacterial sexually transmitted disease, especially among young women worldwide. The aim of this study was comparison the prevalence of Chlamydia trachomatis infection in woman with tubal infertility by means of PCR and cell culture techniques.

METHODS

Fifty-one women with confirmed TFI were enrolled in this study in (avicenna infertility Clinic) between January 2010 and January 2011. Cervical swab and cytobrush specimens were collected from each patient by gynecologists and sent to laboratory in transport media. Detection of Chlamydia trachomatis in samples was performed using PCR and bacteria culture in MacCoy cell line. The data were analyzed by Fisher's exact test and independent t-test. Statistical significance was established at a p-value <0.05.

RESULTS

A significant relation was observed between increased the age of first intercourse and chlamydial infection. Six (11.7%) samples had positive PCR result, whereas cell culture results were positive in only 2 (3.9%) samples. A significant relation was also identified between the duration of infertility and infection (p < 0.05) by PCR versus cell culture method.

CONCLUSION

The results showed that PCR is a rapid method, compared to cell culture for detecting Chlamydial organism. It also became clear that the age at first intercourse is important to predict the likelihood of Chlamydia trachomatis.

Efficacy of Oral Brush Biopsy without Computer-Assisted Analysis in Oral Premalignant and Malignant Lesions: A Study

BACKGROUND

In the present study, the reliability of oral brush biopsy in identifying dysplasia in clinically diagnosed oral potentially malignant and malignant lesions was evaluated while comparing the findings with scalpel biopsy in terms of sensitivity and specificity.

MATERIALS AND METHODS

In our study, a total number of 50 patients that included both premalignant and malignant lesions were included. Oral brush cytology using a cytobrush was done for all patients, which was followed by incisional biopsy. Sensitivity, specificity, positive and negative predictive values were obtained. To see the agreement between two modalities Kappa test of agreement was applied. A P < 0.05 was considered to indicate statistical significance. Proportions were compared using Chi-square or Fisher's exact test.

RESULTS

Brush cytology using a cytobrush is a reliable adjunct to histopathology in detecting oral premalignant and malignant oral lesions and can be easily performed with less cost and less discomfort. This technique showed a reasonable sensitivity and specificity thus substantiating its reliability in evaluation of oral premalignant and malignant lesions.

CONCLUSION

The oral brush biopsy is a simple and rapid, non-invasive and relatively painless and well accepted by patient. It is suitable in population screening programs and for pre- and post-treatment observation of confirmed premalignant and malignant lesions and has proved applications in incapacitated areas.

Uterine microbial profiles in healthy postpartum dairy cows do not vary with sampling techniques or phases of estrous cycle

In this study, we aimed to compare uterine microbial profiles in postpartum dairy cows, determined by bacteriological culture and next-generation sequencing, using three uterine sampling techniques (swab, cytobrush, and lavage) and induced phases of the estrous cycle (estrus and diestrus). Fifteen healthy postpartum dairy cows at 53 ± 5 days postpartum were enrolled in the study. Uterine samples were collected during a fixed-time artificial insemination protocol. Viable bacteria were aerobically cultured from part of each sample, and bacterial isolates were identified through Sanger sequencing of the 16S rRNA gene. Total genomic DNA was extracted from the remainder of undiluted samples to quantify bacterial load using 16S rRNA qPCR and characterize the microbiome by metagenomic sequencing of the V1-V3 region of the 16S rRNA gene. Microbial profiles and composition were analyzed using the Shannon-Weaver diversity index and principal component analysis, respectively. Out of 87 samples, 88 % (77/87) were culture positive. The proportion of culture-positive uterine samples did not differ between sampling techniques (P = 0.39) or estrous cycle phases (P = 0.99). However, swab, cytobrush, and lavage techniques yielded 1.5, 9 and 9 times greater bacterial loads (P < 0.01), respectively, during diestrus than estrus phase. Moreover, during diestrus phase, the cytobrush method yielded 3 and 6 times more bacteria (P < 0.01) than both the lavage and swab methods. The most abundant bacterial genera identified from both bacteriological culture and metagenomic sequencing were Bacillus and Enterococcus, regardless of sampling technique or phases of the estrous cycle. Bacterial genera in moderate to low abundance through metagenomic sequencing included Streptococcus, Oscillospiraceae, and Lachnospiraceae. Notably, the uterine microbial profiles and composition, determined by metagenomic sequencing, did not differ by sampling techniques (P = 0.55 and P = 0.60, respectively) or estrous cycle phases (P = 0.34 and P = 0.17, respectively). In conclusion, our results suggest that any of the sampling techniques can be reliably used to study the uterine microbiome of healthy cows at random phases of the estrous cycle. However, it is important to consider potential differences in bacterial yield as a confounding factor.