Rare diseases

CRP and platelets help diagnose giant cell arteritis

A combination of C-reactive protein (CRP) and platelet count may be useful in the diagnosis of giant cell arteritis (GCA), Australian rheumatologists have shown.

In a retrospective audit of 270 patients referred for temporal artery biopsy (TAB) in South Australian teaching hospitals between 2011 and 2014, 139 (51%) received a physician diagnosis of GCA.

A positive TAB result was reported in only 58% of these patients with other patients diagnosed on the basis of suggestive clinical features and response to glucocorticoid therapy.

Comparing highest recorded values for ESR, CRP and platelets in the two weeks prior to biopsy with diagnoses, the study found the diagnostic accuracy of positive tests was only moderate.

Cut-off values to define a positive test were determined to be 50 mm/hour for ESR, 20 mg/L for CRP, and 300 x 109/L for platelet counts.

“While the AUC [area under the curve] estimate for the ESR (0.65) is slightly less than for CRP (0.72) or platelet counts (0.72), indicating a slightly lower utility of the ESR, the three ROC curves are in fact comparable (p = 0.08), and AUC values in this range indicate only moderate, or borderline acceptable performance as diagnostic tests,” said the study authors, led by Dr Fiona Li Ying Chan and Dr Catherine Hill of the Queen Elizabeth Hospital, Adelaide.

“While each test is significant in individual univariable regression, the multivariable regression demonstrates that both CRP and platelet count are independent predictors of GCA (p < 0.001), whereas the ESR is not (p =0.76).”

“In other words, given CRP and platelet results, the ESR is not informative, and a combination of CRP and platelet results may the most informative for a diagnosis of GCA.”

The study found that CRP and platelet values that were both below the thresholds may be a useful test for the exclusion of GCA (negative predictive value 77, 95% CI 66, 86).

“Conversely, if both CRP and platelet tests are positive, then this may be a useful test for the diagnosis of GCA (positive predictive value 77, 95% CI 67, 85).”

The authors noted that interest was focusing on acute phase reactants to diagnose GCA in addition to traditional criteria such as abnormal temporal artery biopsy or an ESR of 50mm/h or more.

“The postulated mechanism of thrombocytosis in promoting inflammation stems from their early interaction with the endothelium in inflammatory states during which they provide adhesion molecules and chemotactic stimulation to aid in the recruitment of leukocytes and enhance the release of different proinflammatory mediators,” they wrote.

The laboratory markers may provide additional information on which to base a diagnosis especially when the sensitivity of TAB is variable, they concluded.

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