22  Pulmonary Alveolar Proteinosis Syndrome	395
Radiographic Assessment	Laboratory Studies and Biomarkers

The chest radiograph in PAP can be similar in appearance to that of pulmonary edema (diffuse bilateral symmetrical perihilar in ltrates) but without other radiographic features of heart failure [80, 81]. A high-resolution computed tomography of the chest should be performed as the plain chest radiograph lacks sensitivity and speci city. Radiographic ndings on the HRCT include ground-glass opaci cation superimposed on septal thickening with abnormal regions sharply demarcated from more normalappearing regions resulting in what is often referred to as “geographic” appearance. These regions appearing as polygonal shapes and sharply marginated areas of involvement correspond to secondary lobules, interlobular, and lobar boundaries and give an appearance often referred to as “crazy paving” [81, 82]. While this geographic, “crazy paving” pattern is commonly seen in autoimmune PAP, is not diagnostic but, rather, can also be seen in various other idiopathic, infectious, neoplastic and inhalational lung disorders [82]. Neither the chest radiograph nor chest CT is capable of identifying any speci c PAP-causing disease [83].

Bronchoscopy and Bronchoalveolar Lavage

Bronchoalveolar lavage can be useful in diagnosing PAP syndrome and for assessment of secondary lung infection, which should typically be done when performing bronchoscopy in these patients because of the increased infection risk. BAL fuid has a characteristic milky or waxy appearance and can contain large amounts of surfactant sediment that settles if left standing. Microscopic examination of BAL specimens stained with Periodic acid–Schiff reagent or Papanicolaou reveals a “dirty appearing” cytology with acellular eosinophilic globules, large foamy alveolar macrophages (that also appear red after staining with oil-red-O), and signi cant amounts of cell debris (that stains only weakly with PAS) [2, 23, 84, 85]. Biochemical analysis of BAL fuid from patients with PAP shows increased phospholipids, a greater increase in cholesterol (and cholesterol ester) resulting in a markedly increased cholesterol to phospholipid ratio, an increase in surfactant proteins, and increased cytokine biomarkers of PAP (macrophage colony stimulating factor and monocyte chemoattractant protein) [86].

Routine laboratory studies are usually normal in PAP except for serum lactate dehydrogenase (LDH) levels, which are typically elevated in proportion to lung disease severity and A-aD02 [2, 87]. In secondary PAP associated with hematological conditions, other laboratory abnormalities may be present. Other serum biomarkers of PAP include tumor antigens CEA [88], CYFRA 21 [5, 89], and NSE [90], lung epithelium derived proteins (KL-6 [91], SP-A, SP-B, and SP-D [92, 93]), MCP-1 [94] and the chitinase-like protein, YKL-­40 variably correlate with disease severity but none of these are diagnostic of PAP syndrome or any of the PAP-causing diseases [7].

GM-CSF Autoantibodies

An abnormal serum GM-CSF autoantibody test is the most well-established and useful biomarker, partly because an abnormal (increased) level is very speci c for autoimmune PAP [1, 18, 74, 95–97]. While serum GM-CSF autoantibodies are present at non-zero levels in healthy people [73], a high serum level is diagnostic of autoimmune PAP and has a reported sensitivity and speci city of 100% [97, 98].

GM-CSF Signaling Defciency Test

Serum GM-CSF signaling tests are useful in the evaluation of PAP patients and are abnormal in primary (autoimmune and hereditary) PAP but not in secondary or congenital PAP [25, 99–104]. In autoimmune PAP both the serum GM-CSF autoantibody test and a serum GM-CSF signaling test are abnormal. In hereditary PAP, the autoantibody test is normal (negative for a high level of antibodies), but the GM-CSF signaling test is abnormal. In other PAP-causing diseases, both these tests are typically normal.

Genetic Testing

When the GM-CSF autoantibody test is normal and the GM-CSF signaling test is abnormal, screening for mutations in either CSF2RA or CSF2RB can be useful.(REFS) When the GM-CSF autoantibody and GM-CSF signaling tests are normal in a patient with PAP, screening for mutations in genes required for surfactant production-related genetic mutations should be pursued including for the genes encoding SFTPB [105–107], SFTPC [107–109],, ABCA3 [110, 111], or NKX2.1 [112]. Testing for mutations of SLC7A7 and MARS should be considered if there is suspicion of secondary PAP [113–116].