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Correlation between serum total globulins and gamma globulins and their use to diagnose failure of passive transfer in foals
The Veterinary Journal 202 (2014) 384–386
Contents lists available at ScienceDirect
The Veterinary Journal j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / t v j l
Short Communication
Correlation between serum total globulins and gamma globulins and their use to diagnose failure of passive transfer in foals Nathalie Fouché a, Claudia Graubner a, Judith Howard b,* a
Swiss Institute of Equine Medicine (ISME), Vetsuisse-Faculty, University of Bern and ALP-Haras, Länggassstrasse 124, 3001 Bern, Switzerland Clinical Diagnostic Laboratory, Department of Clinical Veterinary Medicine, Vetsuisse-Faculty, University of Bern, Länggassstrasse 124, 3001 Bern, Switzerland b
A R T I C L E
I N F O
Article history: Accepted 13 August 2014 Keywords: Globulins Horse Immunoglobulins Serum electrophoresis Total protein
A B S T R A C T
Various assays have been used as an aid to diagnose failure of passive transfer (FPT) of immunoglobulins in neonatal foals, but often lack sensitivity as screening tests, or are time consuming to perform and impractical as confirmatory tests. The aim of the present study was to evaluate whether measurement of serum total globulins (TG; i.e. total protein minus albumin) can be used to estimate the electrophoretic gamma globulin (EGG) fraction in hospitalised neonatal foals with suspected FPT. Sample data from 56 foals were evaluated retrospectively. The coefficient of rank correlation was 0.84. The area under the curve of ROC analysis was 0.887, 0.922 and 0.930 for EGG concentrations <2 g/L, < 4 g/L and <8 g/L, respectively. Cut-offs for TG achieved ≥90% sensitivity for detecting EGG <2 g/L, < 4 g/L and <8 g/L, with negative predictive values of >97% and >94%, using prevalence of 15% and 30%, respectively. These results suggest that measurement of TG can be used as a guide to predicting EGG, provided that appropriate cut-off values are selected, and this technique could be a useful initial screening test for FPT in foals. © 2014 Elsevier Ltd. All rights reserved.
Failure of passive transfer (FPT) of immunoglobulins in neonatal foals is associated with increased risk of infection and death (Clabough et al., 1989). Previous studies indicate that FPT occurs in 10–20% of foals, rising to 30% in those animals that require hospitalisation (McCue, 2007). Detection of FPT is important in guiding therapeutic decisions, such as the need for colostrum transfer or administration of intravenous plasma. Assays described to diagnose FPT include single radial immunodiffusion (SRID), zinc sulfate turbidity, sodium sulfite precipitation, glutaraldehyde coagulation, serum electrophoresis, latex agglutination and enzyme or turbidimetric immunoassays (Rumbaugh et al., 1978; Clabough et al., 1989; Pusterla et al., 2002; Davis et al., 2005; Davis and Giguère, 2005; McCue, 2007). Although SRID has historically been considered to be the gold standard for diagnosis of FPT, the assay is time consuming to perform, susceptible to error in terms of measurement of the precipitin ring and not practical in many clinical situations that require a rapid diagnosis (Davis et al., 2005; Wong et al., 2013). Furthermore, some degree of variation has been found when comparing assays from different manufacturers (Davis and Giguère, 2005), making it difficult to compare results between research studies and between different diagnostic laboratories. In spite of this variability, FPT has
* Corresponding author. Tel.: +41 31 631 2220. E-mail address: [email protected] (J. Howard). http://dx.doi.org/10.1016/j.tvjl.2014.08.013 1090-0233/© 2014 Elsevier Ltd. All rights reserved.
almost universally been described using cut-off values of <2 g/L or <4 g/L for total FPT and 2–8 g/L or 4–8 g/L for partial FTP (Pusterla et al., 2002; Davis and Giguère, 2005), which is questionable when different assays give different results. This has prompted some authors to recommend other methods, such as immunoturbidimetry, as a gold standard for diagnosis of FPT (Davis et al., 2005; Wong et al., 2013), although the ideal assay to differentiate rapidly and reliably between foals with and without FPT remains to be established. Serum electrophoresis was found to be a satisfactory method to assess circulating IgG in foals compared with SRID, but total protein was considered unreliable (Rumbaugh et al., 1978). Measurement of serum total globulins (TG) has been reported to be a potentially useful screening test, when compared to SRID or immunoturbidimetry (Metzger et al., 2006; Hurcombe et al., 2012). The aim of the present study was to determine the sensitivity and specificity of TG as a predictor of FPT in foals, by comparing this measurement with electrophoretic gamma globulin (EGG) concentrations. The laboratory information database of the Department of Clinical Veterinary Medicine, University of Bern, was interrogated for results of serum electrophoresis performed in hospitalised horses between August 2009 and February 2014. Serum had been analysed by agarose gel electrophoresis (Hydragel 7 Proteins, Sebia) using a semi-automated method (Sebia Hydrasys2, Sebia) described previously (Riond et al., 2009). Animals were included if they were <1 month old, as substantial amounts of endogenous IgG have usually not been produced by this time (Wong et al., 2013). Data were
N. Fouché et al./The Veterinary Journal 202 (2014) 384–386
385
included if serum total protein and albumin concentrations had been measured on the same sample with a clinical chemistry analyser (Roche Cobas c501, Roche Diagnostics) via biuret and bromocresol green dye-binding assays, respectively. Only initial values were included, if more than one electrophoresis was performed on the same foal and post-transfusion data were excluded. The relationship between TG (calculated as total protein minus albumin concentration) and EGG was evaluated using Spearman’s rank correlation. The accuracy of TG to predict EGG was assessed using ROC curve analysis. Data were analysed using commercial software (MedCalc v13.0.6, MedCalc Software). Fifty-six foals were included aged from 0 (<24 h old) to 25 days old (mean of 3.6 days; median of 2.0 days). Twenty-eight were male, 26 were female, and gender was unrecorded in two foals. Fifteen breeds were represented with Swiss Warmblood (n = 34) and Freiberger (n = 6) most prevalent. The concentrations of TG and EGG were 8.0–46.2 g/L (median, 18.3 g/L) and 0.59–23.56 g/L (median, 3.82 g/L), respectively. Foals were divided into groups, based on EGG concentrations, with those considered to have normal passive transfer >8 g/L (n = 13) and classifying those considered to have FPT as mild (4–7.9 g/L; n = 13), moderate (2–3.9 g/L; n = 17) or severe (<2 g/L; n = 13). The concentrations of TG and EGG were significantly correlated, rs = 0.84 (95% confidence interval [CI], 0.73–0.90; P < 0.0001; Fig. 1).
Fig. 1. Scatter plot showing the electrophoretic gamma globulin concentration compared with serum total globulins in 56 foals. Spearman’s coefficient of rank correlation was 0.84 (P < 0.0001, 95% confidence interval 0.73–0.90).
Table 1 Summary of ROC curve analyses for the prediction of gamma-globulin concentrations from total globulin concentrations in 56 foals. Gamma globulins (g/L)
Number of animals Positive group
Negative group
13 30 43
43 26 13
<2.0 <4.0 <8.0
AUC
Standard error
95% CI
P value
0.887 0.922 0.930
0.047 0.036 0.033
0.774 – 0.956 0.819 – 0.977 0.829 – 0.981
<0.0001 <0.0001 <0.0001
AUC, area under the curve; CI, confidence intervals.
Table 2 Specificities, cut-off values and positive and negative likelihood ratios at sensitivities between 90% and 100% for the accuracy of total globulin concentrations to predict electrophoretic gamma-globulin concentrations. Gamma globulins (g/L)
<2.0 <4.0
<8.0
Sensitivity
Specificity
Cut-off
Likelihood ratios
(%)
95% CI
(%)
95% CI
(g/L)
+LR
95% CI
−LR
95% CI
92.31 100.0 90.00 93.33 96.67 100.0 90.70 95.35 97.67 100.0
64.0–99.8 75.3–100.0 73.5–97.9 77.9–99.2 82.8–99.9 88.4–100.0 77.9–97.4 84.2–99.4 87.7–99.9 91.8–100.0
79.07 53.49 76.92 73.08 61.54 15.38 76.92 46.15 23.08 15.38
64.0–90.0 37.7–68.8 56.4–91.0 52.2–88.4 40.6–79.8 4.4–34.9 46.2–95.0 19.2–74.9 5.0–53.8 1.9–45.4
≤16.2 ≤21.0 ≤21.0 ≤21.6 ≤24.9 ≤34.3 ≤26.8 ≤30.3 ≤34.3 ≤40.2
4.41 2.15 3.90 3.47 2.51 1.18 3.93 1.77 1.27 1.18
2.4–8.1 1.6–3.0 1.9–7.9 1.8–6.6 1.5–4.1 1.0–1.4 1.5–10.7 1.1–2.9 0.9–1.7 0.9–1.5
0.097 0.00 0.13 0.091 0.054 0.00 0.12 0.10 0.10 0.00
0.01–0.6 0.04–0.4 0.02–0.4 0.008–0.4 0.05–0.3 0.02–0.4 0.01–0.9
+LR, positive likelihood ratio; − LR, negative likelihood ratio; CI, confidence interval.
Table 3 Positive and negative predictive values of total globulin concentrations to predict electrophoretic gamma-globulin concentrations, using an estimated general and hospital prevalence of 15% and 30%, respectively. Prevalence 15%
30%
Gamma globulins (g/L)
Sensitivity (%)
Specificity (%)
Cut-off (g/L)
PPV
NPV
<2.0 <4.0 <8.0 <2.0 <4.0 <8.0
92.31 90.00 90.70 92.31 90.00 90.70
79.07 76.92 76.92 79.07 76.92 76.92
≤16.2 ≤21.0 ≤26.8 ≤16.2 ≤21.0 ≤26.8
43.8 40.8 41.0 65.4 62.6 62.7
98.3 97.8 97.9 96.0 94.7 95.1
PPV, positive predictive value; NPV, negative predictive value.
386
N. Fouché et al./The Veterinary Journal 202 (2014) 384–386
A summary of ROC curve analyses is presented in Table 1 (see Appendix: Supplementary material for detailed analysis). Cut-off values of TG achieved ≥90% sensitivity for detecting mild, moderate and severe FPT (Table 2). Negative predictive values were >97% and >94% using these cut-offs, with an estimated prevalence in the general and hospitalised foal populations of 15% and 30%, respectively (Table 3). Concentrations of TG ≤22.3 g/L (95% CI, 75.3– 100%) and ≤16.4 g/L (95% CI, 86.8–100%) were 100% specific for mild and moderate FPT, respectively (see Appendix: Supplementary material). In the population tested, this allowed an immediate diagnosis of mild FPT in 36/56 (64%) and moderate FPT in 22/56 (39%) cases. Serum TG measured by this method allow EGG and therefore FPT to be predicted in foals and might be useful as a screening test in clinical practice. The results of the present study compare favourably with reports for other screening tests that have shown sensitivities of 73–100% and specificities of 71–96% for detecting foals at <4 g/L IgG, and sensitivities of 52–98% and specificities of 57– 100% for detecting foals at <8 g/L IgG (Clabough et al., 1989; Davis and Giguère, 2005; Wong et al., 2013). However, cut-off values are valid only for the laboratory in which they were established, as they are specific to the methodology used (analyser and reagents) and cannot be generalised. Nevertheless, in addition to other diagnostic tests, serum/plasma total protein and albumin are frequently measured in hospitalised foals with suspected FPT. Establishing bespoke cut-off values for a specific method would therefore not be associated with much added expense and may allow for better selection of cases in which a confirmatory test is indicated. A previous study (Metzger et al., 2006) suggested that, based on arbitrarily predetermined cut-off values, TG was a poor indicator of FTP, but could be used as an adjunctive test. However, given that the priority for a screening test is to have high sensitivity to allow selection of animals for a confirmation test, the results of both the previous and the present studies suggest that values can be established to do precisely this. Since the consequence of missing a diagnosis of FPT has greater implications than treating an animal unnecessarily, a screening test should aim to identify most if not all affected animals. The high negative predictive value of TG largely excludes FPT when TG is above the cut-off value. In foals with TG below the cut-off, FPT should be confirmed by a more robust assay. Since results of SRID are generally not available for at least 24 h after sample submission, other confirmation tests, such as electrophoresis or immunoturbidimetry, would seem to be most appropriate, possibly with a preference for electrophoresis as it is independent of any kit or internal standard.
Conflict of interest statement None of the authors of this paper has a financial or personal relationship with other people or organisations that could inappropriately influence or bias the content of the paper.
Appendix: Supplementary material Supplementary data to this article can be found online at doi:10.1016/j.tvjl.2014.08.013.
References Clabough, D.L., Conboy, H.S., Roberts, M.C., 1989. Comparison of four screening techniques for the diagnosis of equine neonatal hypoglobulinemia. Journal of the American Veterinary Medical Association 194, 1717–1720. Davis, R., Giguère, S., 2005. Evaluation of five commercially available assays and measurement of serum total protein concentration via refractometry for the diagnosis of failure of passive transfer of immunity in foals. Journal of the American Veterinary Medical Association 227, 1640–1645. Davis, D.G., Shaefer, D.M.W., Hinchcliff, K.W., Wellman, M.L., Willet, V.E., Fletcher, J.M., 2005. Measurement of serum IgG in foals by radial immunodiffusion and automated turbidometric immunoassay. Journal of Veterinary Internal Medicine 19, 93–96. Hurcombe, S.D.A., Matthews, A.L., Scott, V.H.L., Williams, J.M., Kohn, C.W., Toribio, R.E., 2012. Serum protein concentrations as predictors of serum immunoglobulin G concentration in neonatal foals. Journal of Veterinary Emergency and Critical Care 22, 573–579. McCue, P.M., 2007. Evaluation of a turbidometric immunoassay for measurement of plasma IgG concentration in foals. American Journal of Veterinary Research 68, 1005–1009. Metzger, N., Hinchcliff, K.W., Hardy, J., Schwarzwald, C.C., Wittum, T., 2006. Usefulness of a commercial equine IgG test and serum protein concentrations as indicators of failure of transfer of passive immunity in hospitalized foals. Journal of Veterinary Internal Medicine 20, 382–387. Pusterla, N., Pusterla, J.B., Spier, S.J., Puget, B., Watson, J.L., 2002. Evaluation of the SNAP foal IgG test for the semiquantitative measurement of immunoglobulin G in foals. Veterinary Record 151, 258–260. Riond, B., Wenger-Riggenbach, B., Hoffmann-Lehmann, R., Lutz, H., 2009. Serum protein concentrations from clinically healthy horses determined by agarose gel electrophoresis. Veterinary Clinical Pathology 28, 73–77. Rumbaugh, G.E., Ardans, A.A., Ginno, D., Trommershausen-Smith, A., 1978. Measurement of neonatal equine immunoglobulins for assessment of colostral immunoglobulin transfer: Comparison of single radial immunodiffusion with the zinc sulfate turbidity test, serum electrophoresis, refractometry of total serum protein, and the sodium sulfite precipitation test. Journal of the American Veterinary Medical Association 172, 321–325. Wong, D.M., Giguére, S., Wendel, M.A., 2013. Evaluation of a point-of-care portable analyzer for measurement of plasma immunoglobulin G, total protein, and albumin concentrations in ill neonatal foals. Journal of the American Veterinary Medical Association 242, 812–819.
Contents lists available at ScienceDirect
The Veterinary Journal j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / t v j l
Short Communication
Correlation between serum total globulins and gamma globulins and their use to diagnose failure of passive transfer in foals Nathalie Fouché a, Claudia Graubner a, Judith Howard b,* a
Swiss Institute of Equine Medicine (ISME), Vetsuisse-Faculty, University of Bern and ALP-Haras, Länggassstrasse 124, 3001 Bern, Switzerland Clinical Diagnostic Laboratory, Department of Clinical Veterinary Medicine, Vetsuisse-Faculty, University of Bern, Länggassstrasse 124, 3001 Bern, Switzerland b
A R T I C L E
I N F O
Article history: Accepted 13 August 2014 Keywords: Globulins Horse Immunoglobulins Serum electrophoresis Total protein
A B S T R A C T
Various assays have been used as an aid to diagnose failure of passive transfer (FPT) of immunoglobulins in neonatal foals, but often lack sensitivity as screening tests, or are time consuming to perform and impractical as confirmatory tests. The aim of the present study was to evaluate whether measurement of serum total globulins (TG; i.e. total protein minus albumin) can be used to estimate the electrophoretic gamma globulin (EGG) fraction in hospitalised neonatal foals with suspected FPT. Sample data from 56 foals were evaluated retrospectively. The coefficient of rank correlation was 0.84. The area under the curve of ROC analysis was 0.887, 0.922 and 0.930 for EGG concentrations <2 g/L, < 4 g/L and <8 g/L, respectively. Cut-offs for TG achieved ≥90% sensitivity for detecting EGG <2 g/L, < 4 g/L and <8 g/L, with negative predictive values of >97% and >94%, using prevalence of 15% and 30%, respectively. These results suggest that measurement of TG can be used as a guide to predicting EGG, provided that appropriate cut-off values are selected, and this technique could be a useful initial screening test for FPT in foals. © 2014 Elsevier Ltd. All rights reserved.
Failure of passive transfer (FPT) of immunoglobulins in neonatal foals is associated with increased risk of infection and death (Clabough et al., 1989). Previous studies indicate that FPT occurs in 10–20% of foals, rising to 30% in those animals that require hospitalisation (McCue, 2007). Detection of FPT is important in guiding therapeutic decisions, such as the need for colostrum transfer or administration of intravenous plasma. Assays described to diagnose FPT include single radial immunodiffusion (SRID), zinc sulfate turbidity, sodium sulfite precipitation, glutaraldehyde coagulation, serum electrophoresis, latex agglutination and enzyme or turbidimetric immunoassays (Rumbaugh et al., 1978; Clabough et al., 1989; Pusterla et al., 2002; Davis et al., 2005; Davis and Giguère, 2005; McCue, 2007). Although SRID has historically been considered to be the gold standard for diagnosis of FPT, the assay is time consuming to perform, susceptible to error in terms of measurement of the precipitin ring and not practical in many clinical situations that require a rapid diagnosis (Davis et al., 2005; Wong et al., 2013). Furthermore, some degree of variation has been found when comparing assays from different manufacturers (Davis and Giguère, 2005), making it difficult to compare results between research studies and between different diagnostic laboratories. In spite of this variability, FPT has
* Corresponding author. Tel.: +41 31 631 2220. E-mail address: [email protected] (J. Howard). http://dx.doi.org/10.1016/j.tvjl.2014.08.013 1090-0233/© 2014 Elsevier Ltd. All rights reserved.
almost universally been described using cut-off values of <2 g/L or <4 g/L for total FPT and 2–8 g/L or 4–8 g/L for partial FTP (Pusterla et al., 2002; Davis and Giguère, 2005), which is questionable when different assays give different results. This has prompted some authors to recommend other methods, such as immunoturbidimetry, as a gold standard for diagnosis of FPT (Davis et al., 2005; Wong et al., 2013), although the ideal assay to differentiate rapidly and reliably between foals with and without FPT remains to be established. Serum electrophoresis was found to be a satisfactory method to assess circulating IgG in foals compared with SRID, but total protein was considered unreliable (Rumbaugh et al., 1978). Measurement of serum total globulins (TG) has been reported to be a potentially useful screening test, when compared to SRID or immunoturbidimetry (Metzger et al., 2006; Hurcombe et al., 2012). The aim of the present study was to determine the sensitivity and specificity of TG as a predictor of FPT in foals, by comparing this measurement with electrophoretic gamma globulin (EGG) concentrations. The laboratory information database of the Department of Clinical Veterinary Medicine, University of Bern, was interrogated for results of serum electrophoresis performed in hospitalised horses between August 2009 and February 2014. Serum had been analysed by agarose gel electrophoresis (Hydragel 7 Proteins, Sebia) using a semi-automated method (Sebia Hydrasys2, Sebia) described previously (Riond et al., 2009). Animals were included if they were <1 month old, as substantial amounts of endogenous IgG have usually not been produced by this time (Wong et al., 2013). Data were
N. Fouché et al./The Veterinary Journal 202 (2014) 384–386
385
included if serum total protein and albumin concentrations had been measured on the same sample with a clinical chemistry analyser (Roche Cobas c501, Roche Diagnostics) via biuret and bromocresol green dye-binding assays, respectively. Only initial values were included, if more than one electrophoresis was performed on the same foal and post-transfusion data were excluded. The relationship between TG (calculated as total protein minus albumin concentration) and EGG was evaluated using Spearman’s rank correlation. The accuracy of TG to predict EGG was assessed using ROC curve analysis. Data were analysed using commercial software (MedCalc v13.0.6, MedCalc Software). Fifty-six foals were included aged from 0 (<24 h old) to 25 days old (mean of 3.6 days; median of 2.0 days). Twenty-eight were male, 26 were female, and gender was unrecorded in two foals. Fifteen breeds were represented with Swiss Warmblood (n = 34) and Freiberger (n = 6) most prevalent. The concentrations of TG and EGG were 8.0–46.2 g/L (median, 18.3 g/L) and 0.59–23.56 g/L (median, 3.82 g/L), respectively. Foals were divided into groups, based on EGG concentrations, with those considered to have normal passive transfer >8 g/L (n = 13) and classifying those considered to have FPT as mild (4–7.9 g/L; n = 13), moderate (2–3.9 g/L; n = 17) or severe (<2 g/L; n = 13). The concentrations of TG and EGG were significantly correlated, rs = 0.84 (95% confidence interval [CI], 0.73–0.90; P < 0.0001; Fig. 1).
Fig. 1. Scatter plot showing the electrophoretic gamma globulin concentration compared with serum total globulins in 56 foals. Spearman’s coefficient of rank correlation was 0.84 (P < 0.0001, 95% confidence interval 0.73–0.90).
Table 1 Summary of ROC curve analyses for the prediction of gamma-globulin concentrations from total globulin concentrations in 56 foals. Gamma globulins (g/L)
Number of animals Positive group
Negative group
13 30 43
43 26 13
<2.0 <4.0 <8.0
AUC
Standard error
95% CI
P value
0.887 0.922 0.930
0.047 0.036 0.033
0.774 – 0.956 0.819 – 0.977 0.829 – 0.981
<0.0001 <0.0001 <0.0001
AUC, area under the curve; CI, confidence intervals.
Table 2 Specificities, cut-off values and positive and negative likelihood ratios at sensitivities between 90% and 100% for the accuracy of total globulin concentrations to predict electrophoretic gamma-globulin concentrations. Gamma globulins (g/L)
<2.0 <4.0
<8.0
Sensitivity
Specificity
Cut-off
Likelihood ratios
(%)
95% CI
(%)
95% CI
(g/L)
+LR
95% CI
−LR
95% CI
92.31 100.0 90.00 93.33 96.67 100.0 90.70 95.35 97.67 100.0
64.0–99.8 75.3–100.0 73.5–97.9 77.9–99.2 82.8–99.9 88.4–100.0 77.9–97.4 84.2–99.4 87.7–99.9 91.8–100.0
79.07 53.49 76.92 73.08 61.54 15.38 76.92 46.15 23.08 15.38
64.0–90.0 37.7–68.8 56.4–91.0 52.2–88.4 40.6–79.8 4.4–34.9 46.2–95.0 19.2–74.9 5.0–53.8 1.9–45.4
≤16.2 ≤21.0 ≤21.0 ≤21.6 ≤24.9 ≤34.3 ≤26.8 ≤30.3 ≤34.3 ≤40.2
4.41 2.15 3.90 3.47 2.51 1.18 3.93 1.77 1.27 1.18
2.4–8.1 1.6–3.0 1.9–7.9 1.8–6.6 1.5–4.1 1.0–1.4 1.5–10.7 1.1–2.9 0.9–1.7 0.9–1.5
0.097 0.00 0.13 0.091 0.054 0.00 0.12 0.10 0.10 0.00
0.01–0.6 0.04–0.4 0.02–0.4 0.008–0.4 0.05–0.3 0.02–0.4 0.01–0.9
+LR, positive likelihood ratio; − LR, negative likelihood ratio; CI, confidence interval.
Table 3 Positive and negative predictive values of total globulin concentrations to predict electrophoretic gamma-globulin concentrations, using an estimated general and hospital prevalence of 15% and 30%, respectively. Prevalence 15%
30%
Gamma globulins (g/L)
Sensitivity (%)
Specificity (%)
Cut-off (g/L)
PPV
NPV
<2.0 <4.0 <8.0 <2.0 <4.0 <8.0
92.31 90.00 90.70 92.31 90.00 90.70
79.07 76.92 76.92 79.07 76.92 76.92
≤16.2 ≤21.0 ≤26.8 ≤16.2 ≤21.0 ≤26.8
43.8 40.8 41.0 65.4 62.6 62.7
98.3 97.8 97.9 96.0 94.7 95.1
PPV, positive predictive value; NPV, negative predictive value.
386
N. Fouché et al./The Veterinary Journal 202 (2014) 384–386
A summary of ROC curve analyses is presented in Table 1 (see Appendix: Supplementary material for detailed analysis). Cut-off values of TG achieved ≥90% sensitivity for detecting mild, moderate and severe FPT (Table 2). Negative predictive values were >97% and >94% using these cut-offs, with an estimated prevalence in the general and hospitalised foal populations of 15% and 30%, respectively (Table 3). Concentrations of TG ≤22.3 g/L (95% CI, 75.3– 100%) and ≤16.4 g/L (95% CI, 86.8–100%) were 100% specific for mild and moderate FPT, respectively (see Appendix: Supplementary material). In the population tested, this allowed an immediate diagnosis of mild FPT in 36/56 (64%) and moderate FPT in 22/56 (39%) cases. Serum TG measured by this method allow EGG and therefore FPT to be predicted in foals and might be useful as a screening test in clinical practice. The results of the present study compare favourably with reports for other screening tests that have shown sensitivities of 73–100% and specificities of 71–96% for detecting foals at <4 g/L IgG, and sensitivities of 52–98% and specificities of 57– 100% for detecting foals at <8 g/L IgG (Clabough et al., 1989; Davis and Giguère, 2005; Wong et al., 2013). However, cut-off values are valid only for the laboratory in which they were established, as they are specific to the methodology used (analyser and reagents) and cannot be generalised. Nevertheless, in addition to other diagnostic tests, serum/plasma total protein and albumin are frequently measured in hospitalised foals with suspected FPT. Establishing bespoke cut-off values for a specific method would therefore not be associated with much added expense and may allow for better selection of cases in which a confirmatory test is indicated. A previous study (Metzger et al., 2006) suggested that, based on arbitrarily predetermined cut-off values, TG was a poor indicator of FTP, but could be used as an adjunctive test. However, given that the priority for a screening test is to have high sensitivity to allow selection of animals for a confirmation test, the results of both the previous and the present studies suggest that values can be established to do precisely this. Since the consequence of missing a diagnosis of FPT has greater implications than treating an animal unnecessarily, a screening test should aim to identify most if not all affected animals. The high negative predictive value of TG largely excludes FPT when TG is above the cut-off value. In foals with TG below the cut-off, FPT should be confirmed by a more robust assay. Since results of SRID are generally not available for at least 24 h after sample submission, other confirmation tests, such as electrophoresis or immunoturbidimetry, would seem to be most appropriate, possibly with a preference for electrophoresis as it is independent of any kit or internal standard.
Conflict of interest statement None of the authors of this paper has a financial or personal relationship with other people or organisations that could inappropriately influence or bias the content of the paper.
Appendix: Supplementary material Supplementary data to this article can be found online at doi:10.1016/j.tvjl.2014.08.013.
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