Prior probability (the pretest best guess) affects predictive values of diagnostic tests

Comparison of Two Diagnostic Assays for the Detection of Serum Neutralizing Antibody to Porcine Epidemic Diarrhea Virus

Lactogenic immunity is important for the protection of piglets against many pathogens including porcine epidemic diarrhea virus. Circulating neutralizing antibodies levels in sow sera may help determine if a detectable immune response could confer protection to piglets. Neutralizing antibodies can be detected through various diagnostic assays. This study evaluated the diagnostic characteristics of two neutralizing antibody assays for porcine epidemic diarrhea virus neutralizing antibodies in serum of challenged gilts. Four treatment groups, control, non-vaccinated, vaccinated prior to challenge, and vaccinated following challenge, were comprised of 20 gilts. Serum sample were collected from each gilt prior to and following challenge with porcine epidemic diarrhea virus. Samples were evaluated for the presence of neutralizing antibodies via a fluorescent focus neutralization assay and a high-throughput neutralization assay. Diagnostic sensitivity and specificity for the fluorescent focus neutralization and high-throughput neutralization assays for this study were optimized at a cutoff of a dilution of 80 and 80% fluorescent reduction respectively and demonstrated moderate agreement based off the kappa statistic. The focus fluorescent neutralization and high-throughput neutralization assays can be used to monitor the status of neutralizing antibodies within animals or a population of animals. The high-throughput assay has advantages over the focus fluorescent assay in that it has a higher specificity at the indicated cut-off and the nature of the results allows for more discrimination between individual results.

Changes in the Epidemiology of Zoonotic Infections in Children: A Nationwide Register Study in Finland

OBJECTIVES

Zoonotic infections are difficult to recognize in children. The age distributions and seasonal occurrences of these infections vary substantially, even among those transmitted by the same vectors, and their epidemiology may change over time. The aim was to report the incidences and trends of Borrelia burgdorferi, Puumala virus, Francisella tularensis and tick-borne encephalitis (TBE) virus infections in the pediatric population (age 0-19) of Finland.

METHODS

A nationwide survey based on the National Infectious Disease Register was conducted from 1996 to 2019 and all laboratory-confirmed cases were included. Age-stratified incidences per 100,000 person-years were calculated.

RESULTS

Cumulative incidences were B. burgdorferi 11.2, TBE 0.4, Puumala virus 6.4 and F. tularensis 2.5 per 100,000 person-years. An increasing trend in the incidences of B. burgdorferi and TBE was observed. Borrelia expanded geographically northward and inland. Tularemia follows a 2-4-year epidemic cycle and rates are similar across age groups. Puumala incidences are highest in the older children.

DISCUSSION

Borrelia infections increased most rapidly in children 5-9 years of age and overall expanded geographically in Finland. Tularemia epidemic cycles were shorter than those previously reported. These results will help clinicians to identify these infections in different geographic areas and age groups in Finland.

DIAGNOSTIC PERFORMANCE OF INFLAMMATORY MARKERS IN GOPHER TORTOISES ( GOPHERUS POLYPHEMUS)

The lack of sensitive and specific markers of inflammation poses a diagnostic challenge in sick or injured reptile patients. The objective of this study was to investigate the diagnostic performance of blood analytes associated with inflammation in healthy ( n = 24) and sick ( n = 38) gopher tortoises ( Gopherus polyphemus). Receiver operating characteristic analysis identified the following as the best-performing diagnostic tests: erythrocyte sedimentation rate (area under the curve [AUC] = 0.812; 95% confidence interval [CI] = 0.693, 0.900), absolute mature heterophils (AUC = 0.771; 95% CI = 0.646, 0.869), total leukocytes (AUC = 0.767; 95% CI = 0.642, 0.866), lactate (AUC = 0.766; 95% CI = 0.641, 0.864), and absolute immature heterophils (AUC = 0.755; 95% CI = 0.628, 0.856). These results support the clinical application of additional tools for the diagnosis and monitoring of inflammatory disease in gopher tortoises. Clinicians may consider adding erythrocyte sedimentation rate and lactate to the minimum database for this species.

Reporting results with (Un)certainty

BACKGROUND

A degree of uncertainty occurs with every measured laboratory result due to both analytical and biological variation. The tools of Total Observed error (TE_O ) and dispersion based on biological variation have helped veterinary labs quantify the causes of variation that lead to measurement uncertainty (MU). International organizations recommend that the amount of MU in veterinary laboratory results be identified and communicated. The expanded measurement uncertainty (EMU), dispersion, and reporting interval adjustment have been recommended as tools to allow communication of MU to laboratory data users but are not commonly discussed in the veterinary literature.

OBJECTIVE

Using the vocabulary of Total Observed error and biological variation and examples from veterinary medicine, a review of the theory and application of the EMU, dispersion, and the methods for deriving an appropriate reporting interval recommended by Hawkins and Badrick, is presented.

CONCLUSIONS

By addressing the way that MU is communicated to users of laboratory results, the laboratory enables users to better understand the potential uncertainty associated with reported results, helps to prevent over and under-interpretation of data, and improves diagnostic accuracy and patient care.

Diagnostic accuracy of diagnostic imaging for lumbar disc herniation in adults with low back pain or sciatica is unknown; a systematic review

Main text

We aim to summarize the available evidence on the diagnostic accuracy of imaging (index test) compared to surgery (reference test) for identifying lumbar disc herniation (LDH) in adult patients.For this systematic review we searched MEDLINE, EMBASE and CINAHL (June 2017) for studies that assessed the diagnostic accuracy of imaging for LDH in adult patients with low back pain and surgery as the reference standard. Two review authors independently selected studies, extracted data and assessed risk of bias. We calculated summary estimates of sensitivity and specificity using bivariate analysis, generated linked ROC plots in case of direct comparison of diagnostic imaging tests and assessed the quality of evidence using the GRADE-approach.We found 14 studies, all but one done before 1995, including 940 patients. Nine studies investigated Computed Tomography (CT), eight myelography and six Magnetic Resonance Imaging (MRI). The prior probability of LDH varied from 48.6 to 98.7%. The summary estimates for MRI and myelography were comparable with CT (sensitivity: 81.3% (95%CI 72.3-87.7%) and specificity: 77.1% (95%CI 61.9-87.5%)). The quality of evidence was moderate to very low.

Conclusions

The diagnostic accuracy of CT, myelography and MRI of today is unknown, as we found no studies evaluating today's more advanced imaging techniques. Concerning the older techniques we found moderate diagnostic accuracy for all CT, myelography and MRI, indicating a large proportion of false positives and negatives.

Heat treatment and false-positive heartworm antigen testing in ex vivo parasites and dogs naturally infected by Dirofilaria repens and Angiostrongylus vasorum

Background

Heartworm antigen testing is considered sensitive and specific. Currently available tests are reported as detecting a glycoprotein found predominantly in the reproductive tract of the female worm and can reach specificity close to 100%. Main concerns regard sensitivity in the case of light infections, the presence of immature females or cases of all-male infections. Research and development have been aimed at increasing sensitivity. Recently, heat treatment of serum prior to antigen testing has been shown to result in an increase in positive antigen test results, presumably due to disruption of natural antigen–antibody complexes. Cross-reactions in dogs with both natural and experimental infections with Angiostrongylus vasorum and Spirocerca lupi have been reported, but cross-reactions with other helminths have not been extensively studied. In order to evaluate potential cross-reactivity with other canine and feline parasites, two studies were performed. Study 1: Live adults of Dirofilaria immitis, Dirofilaria repens, Toxocara canis, Toxocara cati, Dipylidium caninum, Taenia taeniaeformis and Mesocestoides spp. larvae were washed and incubated in tubes with saline solution. All worms were alive at the time of removal from the saline. Saline solutions containing excretory/secretory antigens were then tested for heartworm with six different, commercially available antigen tests. All results were evaluated blind by three of the authors. Study 2: Sera from dogs with natural infections by A. vasorum or D. repens, living in areas free of heartworm disease, were tested with the same tests before and after heat treatment (103 °C for 10 min).

Results

Results suggest that antigens detected by currently available tests are not specific for D. immitis. They may give positive results through detection of different parasites’ antigens that are normally not released into the bloodstream or released in a low amount and/or bound to antibodies. Tests may even detect antigens released by male D. immitis adult worms. D. repens appears to release more detectable antigens than the other worms studied.

Conclusions

Cross-reaction with A. vasorum and D. repens does occur in the field and could potentially occur with other helminths. Heat treatment decreases specificity by enhancing cross-reactivity.

Diagnostic accuracy of two point-of-care kits for the diagnosis of Giardia species infection in dogs

OBJECTIVES

The objective of this study was to compare results obtained by ZnSO₄ Flotation and SNAP ^®Giardia to those generated by the new point‐of‐care tests Single and Triple Rapid.

METHODS

Prospective study evaluating 51 canine faecal samples submitted at a reference laboratory for the presence of Giardia spp. Kappa statistics, specificity, sensitivity, positive predictive value (PPV) and negative predictive value (NPV) were calculated by comparing the new tests to the combined results of ZnSO₄ and SNAP tests.

RESULTS

There was fair (Single Rapid, j=0·434) to good (Triple Rapid, j =0·797) agreement with the reference tests. At this study's prevalence (59 to 61%), specificities and PPV were high (1·00) with both Rapid tests, but sensitivities and NPV were lower for the Single than for the Triple (0·48 vs 0·83 and 0·55 vs 0·80) tests. At lower prevalence rates, both tests exhibited a high PPV (1·00), but the NPV were higher with the Triple (0·96 to 0·99) than the Single (0·88 to 0·96) Rapid test.

CLINICAL SIGNIFICANCE

Both tests exhibited excellent PPV values at all prevalence rates but an excellent NPV only at low prevalence. As the prevalence is likely to be low (<15%) in clinical settings, we propose that these tests may be helpful in the in‐house diagnosis of Giardia spp infection. However, they exhibit lower sensitivity than the combined sensitivity of ZnSO₄ and SNAP tests, particularly in high prevalence settings.

Diagnostic accuracy of Wright-Giemsa and rhodanine stain protocols for detection and semi-quantitative grading of copper in canine liver aspirates

BACKGROUND

Canine hepatic copper content has been increasing. Recognition of canine copper-associated hepatopathies is becoming more common.

OBJECTIVES

The purpose of the study was to assess the diagnostic performance of Wright-Giemsa (WG) and rhodanine staining for detection of increased canine hepatic copper following a proposed cytologic protocol for semi-quantitative evaluation of liver aspirates and the effect of previous WG staining.

METHODS

Retrospectively, 40 canine hepatic WG-stained cytology cases were rhodanine stained. Diagnostic performance of WG staining for increased hepatic copper was evaluated. A rhodanine-stained cytologic copper grading system was developed. Prospectively, 67 canine liver samples with quantitative copper measurement, a WG-then rhodanine-stained slide, and a non-WG rhodanine-stained slide were used to assess the performance of the grading system and the effect of previous WG staining.

RESULTS

Copper was not described in 40 retrospective cases on initial cytologic evaluation; 8/40 cases had increased copper content after rhodanine staining or quantitative copper assessment. Prior WG staining and destaining significantly affected the cytologic copper grade but not the diagnostic performance as measured by receiver-operating characteristic curve analysis. Quantitative copper concentration and previously WG-stained copper grade were moderately correlated (n = 67, ρ = .79 [.68-.87]). For detection of ≥ 600 ppm, dry weight (dw) copper, sensitivity was .75 and specificity was .97. For detection of ≥ 1500 ppm, dw copper, sensitivity was 1.0 and specificity was .97.

CONCLUSIONS

Wright-Giemsa staining alone does not reliably detect hepatic copper. Grading of rhodanine-stained canine hepatic cytologic samples demonstrates acceptable diagnostic performance for detection of copper content.

Simplified methods for estimating glomerular filtration rate in cats and for detection of cats with low or borderline glomerular filtration rate

OBJECTIVES

Diagnosis of early feline chronic kidney disease (CKD) is challenging. Glomerular filtration rate (GFR) is the best overall indicator of kidney function, but multisample plasma clearance methods to determine GFR are labour intensive, time consuming and stressful for feline patients. This study aimed to develop simplified methods to detect decreased GFR in cats.

METHODS

Data from a nine-sample combined plasma exogenous creatinine-iohexol clearance test of 73 cats were used. Limited sampling strategies were developed by comparing all sampling time combinations with the complete nine sampling times set and selecting the best sampling time combinations based on maximum relative error. By regression analysis, the ability of routine blood (serum creatinine, serum urea) and urine (urine specific gravity, urinary protein:creatinine ratio) variables to predict GFR or identify cats with low or borderline GFR was examined. Cut-off clearance marker concentrations to predict low or borderline GFR was determined at three time points after marker injection. All procedures were analysed for three clearance markers (exo-iohexol, creatinine, endo-iohexol).

RESULTS

For reliable estimation of GFR, at least three blood samples for clinical purposes and five blood samples for research purposes are required. Regression formulae based on routine variables did not reliably predict GFR, but accurately identified cats with low (sensitivity 96.5-98.2%; specificity 60-91.3%) or borderline (sensitivity 91.1-96%; specificity 76.5-81.8%) GFR. Clearance marker concentrations exceeding given marker cut-off concentrations also identified cats with low or borderline GFR with high sensitivities and specificities.

CONCLUSIONS AND RELEVANCE

These simplified methods will facilitate the detection of early kidney dysfunction in cats. Early diagnosis allows timely therapeutic intervention, and future studies must reveal whether this improves the long-term outcome of cats with CKD.

Positive predictive value of albumin: globulin ratio for feline infectious peritonitis in a mid-western referral hospital population

Low albumin to globulin ratio has been found previously to have a high positive predictive value for feline infectious peritonitis (FIP) in cats with clinical signs highly suggestive of the disease. However, FIP can have a more vague clinical presentation. This retrospective study found that the positive predictive value of an albumin:globulin (A:G) ratio of <0.8 and <0.6 was only 12.5% and 25%, respectively, in a group of 100 cats with one or more clinical signs consistent with FIP. The negative predictive value was 100% and 99% for an A:G ratio of <0.8 and A:G<0.6%, respectively. Therefore, when the prevalence of FIP is low, the A:G ratio is useful to rule out FIP but is not helpful in making a positive diagnosis of FIP.